Adding Flash Attention 2 Support for GPT2 (#29226)

* First commit to add flash attention 2 for GPT-2

* more improvements

* Make GPT2 pass tests and fixed Decison Transformers copies

* Fixed missing arg

* fix copies

* Added expected speedup

* Update src/transformers/models/gpt2/modeling_gpt2.py

Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>

* Update src/transformers/models/gpt2/modeling_gpt2.py

Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>

* Update src/transformers/models/gpt2/modeling_gpt2.py

Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>

* Added test

* Fixed attn attribute

* Update docs/source/en/model_doc/gpt2.md

Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>

* Update docs/source/en/model_doc/gpt2.md

Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>

* Update Decision transformer attentions

* More updates

* Passing tests

* Fix copies

* Fix copies part 2

* Decision transformer updates

* Update src/transformers/models/gpt2/modeling_gpt2.py

Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>

* Fix copies

* Decision transformer not supporting flash attn

* Addressed comments

* Addressed comments

* Addressed comments

---------

Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>

docs/source/en/model_doc/gpt2.md

@@ -60,6 +60,73 @@ This model was contributed by [thomwolf](https://huggingface.co/thomwolf). The o
 - Enabling the *scale_attn_by_inverse_layer_idx* and *reorder_and_upcast_attn* flags will apply the training stability
   improvements from [Mistral](https://github.com/stanford-crfm/mistral/) (for PyTorch only).
 
+## Usage example
+
+The `generate()` method can be used to generate text using GPT2 model.
+
+```python
+>>> from transformers import AutoModelForCausalLM, AutoTokenizer
+
+>>> model = AutoModelForCausalLM.from_pretrained("gpt2")
+>>> tokenizer = AutoTokenizer.from_pretrained("gpt2")
+
+>>> prompt = "GPT2 is a model developed by OpenAI."
+
+>>> input_ids = tokenizer(prompt, return_tensors="pt").input_ids
+
+>>> gen_tokens = model.generate(
+...     input_ids,
+...     do_sample=True,
+...     temperature=0.9,
+...     max_length=100,
+... )
+>>> gen_text = tokenizer.batch_decode(gen_tokens)[0]
+```
+
+## Using Flash Attention 2
+
+Flash Attention 2 is a faster, optimized version of the attention scores computation which relies on `cuda` kernels.
+
+### Installation 
+
+First, check whether your hardware is compatible with Flash Attention 2. The latest list of compatible hardware can be found in the [official documentation](https://github.com/Dao-AILab/flash-attention#installation-and-features). If your hardware is not compatible with Flash Attention 2, you can still benefit from attention kernel optimisations through Better Transformer support covered [above](https://huggingface.co/docs/transformers/main/en/model_doc/bark#using-better-transformer).
+
+Next, [install](https://github.com/Dao-AILab/flash-attention#installation-and-features) the latest version of Flash Attention 2:
+
+```bash
+pip install -U flash-attn --no-build-isolation
+```
+
+### Usage
+
+To load a model using Flash Attention 2, we can pass the argument `attn_implementation="flash_attention_2"` to [`.from_pretrained`](https://huggingface.co/docs/transformers/main/en/main_classes/model#transformers.PreTrainedModel.from_pretrained). We'll also load the model in half-precision (e.g. `torch.float16`), since it results in almost no degradation to audio quality but significantly lower memory usage and faster inference:
+
+```python
+>>> import torch
+>>> from transformers import AutoModelForCausalLM, AutoTokenizer
+>>> device = "cuda" # the device to load the model onto
+
+>>> model = AutoModelForCausalLM.from_pretrained("gpt2", torch_dtype=torch.float16, attn_implementation="flash_attention_2")
+>>> tokenizer = AutoTokenizer.from_pretrained("gpt2")
+
+>>> prompt = "def hello_world():"
+
+>>> model_inputs = tokenizer([prompt], return_tensors="pt").to(device)
+>>> model.to(device)
+
+>>> generated_ids = model.generate(**model_inputs, max_new_tokens=100, do_sample=True)
+>>> tokenizer.batch_decode(generated_ids)[0]
+```
+
+
+### Expected speedups
+
+Below is an expected speedup diagram that compares pure inference time between the native implementation in transformers using `gpt2` checkpoint and the Flash Attention 2 version of the model using a sequence length of 512.
+
+<div style="text-align: center">
+<img src="https://huggingface.co/datasets/EduardoPacheco/documentation-images/resolve/main/gpt2_flash_attention_2_speedup.jpg">
+</div>
+
 ## Resources
 
 A list of official Hugging Face and community (indicated by 🌎) resources to help you get started with GPT2. If you're interested in submitting a resource to be included here, please feel free to open a Pull Request and we'll review it! The resource should ideally demonstrate something new instead of duplicating an existing resource.