[docs] update attention implementation and cache docs

docs/source/en/attention_interface.md

@@ -72,6 +72,34 @@ model(torch.ones(1, 5, dtype=int))
 and it will stop printing the statements, as it now uses the `sdpa` attention.  
 This allows to quickly change an attention function, without needing to reload the model!
 
+## Different attention per backbone in multimodal models
+
+For multimodal models, you may want to load different backbones with different attention functions. For example, some vision backbones perform better with full precision only and are incompatible with Flash Attention. If you want to take advantage of Flash Attention while keeping your vision encoder in fp32, you can configure different attention implementations per backbone.
+
+```python
+from transformers import AutoModelForImageTextToText
+
+model_id = "facebook/chameleon-7b"
+
+attention_implementation_per_backbone = {"vision_config": "sdpa", "text_config": "flash_attention_2"}
+model = AutoModelForImageTextToText.from_pretrained(model_id, attn_implementation=attention_implementation_per_backbone)
+
+# NOTE: keys in the attention implementation have to be the same as the sub-config names
+for key in attention_implementation_per_backbone:
+    assert key in model.config.sub_configs, f"Invalid key in `attention_implementation`"
+
+# You can omit certain backbones - the default attention function (SDPA) will be used
+# This is equivalent to the previous example
+model = AutoModelForImageTextToText.from_pretrained(model_id, attn_implementation={"text_config": "flash_attention_2"})
+
+
+# Set the same attention implementation for all backbones with single string, same as in non-multimodal models
+model = AutoModelForImageTextToText.from_pretrained(model_id, attn_implementation="eager")
+
+# Alternatively use a dict with an empty key for global configuration
+model = AutoModelForImageTextToText.from_pretrained(model_id, attn_implementation={"": "eager"})
+```
+
 ## What about new args needed in my custom attention function?
 
 But indeed, what if the new function requires a new arg to be properly used? It's no issue! Models supporting the

docs/source/en/cache_explanation.md

@@ -132,6 +132,35 @@ for _ in range(max_new_tokens):
 print(tokenizer.batch_decode(generated_ids, skip_special_tokens=True)[0])
 "[INST] Hello, what's your name. [/INST]  Hello! My name is LLaMA,"
 ```
+
+### Cache Position Usage
+
+The cache position tracks where the new tokens should be inserted in the attention cache. It represents the *absolute* position of each token in the context, independent of padding or batch structure. Suppose that you have already cached `N` tokens and you are now processing `K` new tokens. Then the cache position for the new tokens will range from `N` to `N + K - 1`. In other words, you're processing tokens at positions - `[N, N + 1, N + 2, ..., N + K - 1]`
+
+Cache position is used internally for two purposes:
+
+1. Selecting new tokens to process in the input sequence and ensuring only tokens that haven’t been cached yet are passed to the model's `forward`.
+2. Storing key/value pairs at the correct positions in the cache. Especially important for fixed-size caches like [`StaticCache`] that preallocate certain length for cache in advance.
+
+Usually you don't have to worry about cache position as the generation loop takes care of it. Yet, in cases where you are writing your own custom generation method, it is important that cache positions are accurate since they are used to write and read key/value states into fixed slots.
+
+There is an example usage with simple custom generation loop above, so let's create a more complex example here. 
+
+```py
+import torch
+from transformers import AutoTokenizer, AutoModelForCausalLM, DynamicCache
+
+model_id = "meta-llama/Llama-2-7b-chat-hf"
+model = AutoModelForCausalLM.from_pretrained(model_id, torch_dtype=torch.bfloat16, device_map="cuda:0")
+tokenizer = AutoTokenizer.from_pretrained(model_id)
+
+messages = [{"role": "user", "content": "You are a helpful assistant."}]
+inputs = tokenizer.apply_chat_template(messages, add_generation_prompt=True, return_tensors="pt", return_dict=True).to("cuda:0")
+generated_ids = model.generate(**inputs, use_cache=True, max_new_tokens=10)
+
+```
+
+
 ## Legacy cache format
 
 Before the [`Cache`] class, the cache used to be stored as a tuple of tuples of tensors. This format is dynamic because it grows as text is generated, similar to [`DynamicCache`].
@@ -157,4 +186,4 @@ generation_outputs = model.generate(**inputs, return_dict_in_generate=True, retu
 
 cache = DynamicCache.from_legacy_cache(generation_outputs.past_key_values)
 legacy_format_cache = cache.to_legacy_cache()
-```
+```

docs/source/en/llm_optims.md

@@ -341,7 +341,7 @@ A known issue with transformer models is that the self-attention mechanism grows
 
 FlashAttention and [FlashAttention-2](./perf_infer_gpu_one#flashattention-2) break up the attention computation into smaller chunks and reduces the number of intermediate read/write operations to the GPU memory to speed up inference. FlashAttention-2 improves on the original FlashAttention algorithm by also parallelizing over sequence length dimension and better partitioning work on the hardware to reduce synchronization and communication overhead.
 
-To use FlashAttention-2, set [attn_implementation](https://hf.co/docs/transformers/main/en/main_classes/text_generation#transformers.PreTrainedModel.from_pretrained.attn_implementation) to `"flash_attention_2"` in [`~PreTrainedModel.from_pretrained`].
+To use FlashAttention-2, set [attn_implementation](https://hf.co/docs/transformers/main/en/main_classes/text_generation#transformers.PreTrainedModel.from_pretrained.attn_implementation) to `"flash_attention_2"` in [`~PreTrainedModel.from_pretrained`] or set with `model.set_attention_implementation("flash_attention_2")` to dynamically update the [attention interface](./attention_interface) after the model is loaded.
 
 ```py
 from transformers import AutoModelForCausalLM, BitsAndBytesConfig
@@ -360,7 +360,7 @@ model = AutoModelForCausalLM.from_pretrained(
 Scaled dot product attention (SDPA) is automatically enabled in PyTorch 2.0 and it supports FlashAttention, xFormers, and PyTorch's C++ implementation. SDPA chooses the most performant attention algorithm if you're using a CUDA backend. For other backends, SDPA defaults to the PyTorch C++ implementation.
 
 > [!TIP]
-> SDPA automaticallysupports FlashAttention-2 as long as you have the latest PyTorch version installed.
+> SDPA automatically supports FlashAttention-2 as long as you have the latest PyTorch version installed.
 
 Use the [torch.nn.attention.sdpa_kernel](https://pytorch.org/docs/stable/generated/torch.nn.attention.sdpa_kernel.html) context manager to explicitly enable or disable any of the four attention algorithms. For example, use `SDPBackend.FLASH_ATTENTION` to enable FlashAttention.
 

docs/source/en/perf_infer_gpu_one.md

@@ -175,7 +175,7 @@ There are three supported implementations available.
 - [xFormers](https://github.com/facebookresearch/xformers) or Memory-Efficient Attention is able to support models with the fp32 torch type.
 - C++ implementation of scaled dot product attention
 
-SDPA is used by default for PyTorch v2.1.1. and greater when an implementation is available. You could explicitly enable SDPA by setting `attn_implementation="sdpa"` in [`~PreTrainedModel.from_pretrained`] though. Certain attention parameters, such as `head_mask` and `output_attentions=True`, are unsupported and returns a warning that Transformers will fall back to the (slower) eager implementation.
+SDPA is used by default for PyTorch v2.1.1. and greater when an implementation is available. You could explicitly enable SDPA by setting `attn_implementation="sdpa"` in [`~PreTrainedModel.from_pretrained`] though. Certain attention parameters, such as `head_mask` and `output_attentions=True`, are unsupported and returns a warning that Transformers will fall back to the (slower) eager implementation. You can also change the attention implementation after loading the model for most architectures. For details, check out the [attention interfaces documentation](./attention_interface).
 
 ```py
 from transformers import AutoModelForCausalLM
@@ -234,7 +234,7 @@ FlashAttention2 support is currently limited to Instinct MI210, Instinct MI250 a
 </hfoption>
 </hfoptions>
 
-Enable FlashAttention2 by setting `attn_implementation="flash_attention_2"` in [`~PreTrainedModel.from_pretrained`]. FlashAttention2 is only supported for models with the fp16 or bf16 torch type. Make sure to cast your model to the appropriate data type first.
+Enable FlashAttention2 by setting `attn_implementation="flash_attention_2"` in [`~PreTrainedModel.from_pretrained`] or by setting `model.set_attention_implementation("flash_attention_2")` to dynamically update the [attention interface](./attention_interface). FlashAttention2 is only supported for models with the fp16 or bf16 torch type. Make sure to cast your model to the appropriate data type first.
 
 ```py
 from transformers import AutoModelForCausalLM