Add GPT-2 with flash attention (#1889)

# What does this PR do?

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This change adds `FlashGPT2ForCausalLM` and wires it up. The model
itself is pretty straightforward, the main difference from other models
is that it uses trained position embeddings and that all weight matrices
are transposed compared to other models (due to the use of Conv1D in the
upstream model).


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Fixes # (issue)


## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the
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      Pull Request section?
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docs/source/supported_models.md

@@ -9,6 +9,7 @@ The following models are optimized and can be served with TGI, which uses custom
 - [BLOOM](https://huggingface.co/bigscience/bloom)
 - [FLAN-T5](https://huggingface.co/google/flan-t5-xxl)
 - [Galactica](https://huggingface.co/facebook/galactica-120b)
+- [GPT-2](https://huggingface.co/openai-community/gpt2)
 - [GPT-Neox](https://huggingface.co/EleutherAI/gpt-neox-20b)
 - [Llama](https://github.com/facebookresearch/llama)
 - [OPT](https://huggingface.co/facebook/opt-66b)

integration-tests/models/__snapshots__/test_flash_gpt2/test_flash_gpt2.json

@@ -0,0 +1,99 @@
+{
+  "details": {
+    "best_of_sequences": null,
+    "finish_reason": "length",
+    "generated_tokens": 10,
+    "prefill": [
+      {
+        "id": 2061,
+        "logprob": null,
+        "text": "What"
+      },
+      {
+        "id": 318,
+        "logprob": -3.1835938,
+        "text": " is"
+      },
+      {
+        "id": 2769,
+        "logprob": -9.171875,
+        "text": " deep"
+      },
+      {
+        "id": 4673,
+        "logprob": -1.6425781,
+        "text": " learning"
+      },
+      {
+        "id": 30,
+        "logprob": -0.7314453,
+        "text": "?"
+      }
+    ],
+    "seed": null,
+    "tokens": [
+      {
+        "id": 198,
+        "logprob": -0.68603516,
+        "special": false,
+        "text": "\n"
+      },
+      {
+        "id": 198,
+        "logprob": -0.005393982,
+        "special": false,
+        "text": "\n"
+      },
+      {
+        "id": 29744,
+        "logprob": -0.31079102,
+        "special": false,
+        "text": "Deep"
+      },
+      {
+        "id": 4673,
+        "logprob": -0.08300781,
+        "special": false,
+        "text": " learning"
+      },
+      {
+        "id": 318,
+        "logprob": -0.58984375,
+        "special": false,
+        "text": " is"
+      },
+      {
+        "id": 257,
+        "logprob": -0.953125,
+        "special": false,
+        "text": " a"
+      },
+      {
+        "id": 649,
+        "logprob": -2.0957031,
+        "special": false,
+        "text": " new"
+      },
+      {
+        "id": 2214,
+        "logprob": -1.8095703,
+        "special": false,
+        "text": " field"
+      },
+      {
+        "id": 286,
+        "logprob": -1.0673828,
+        "special": false,
+        "text": " of"
+      },
+      {
+        "id": 2267,
+        "logprob": -0.9375,
+        "special": false,
+        "text": " research"
+      }
+    ],
+    "top_tokens": null
+  },
+  "generated_text": "\n\nDeep learning is a new field of research"
+}