def test_gpt2_generation(model_name, rotary, optimized):
    """Check that our implementation of GPT2 generation matches the HF implementation:
    the scores in fp16 should be around the same as the HF scores in fp16, when compared to
    the HF scores in fp32.
    """
    dtype = torch.float16
    device = "cuda"
    rtol, atol = 3e-3, 3e-1
    config = GPT2Config.from_pretrained(model_name)
    if rotary:
        config.n_positions = 0
        config.rotary_emb_fraction = 0.5
        config.rotary_emb_base = 24000
    config.residual_in_fp32 = True
    if optimized:
        config.use_flash_attn = True
        config.fused_bias_fc = True
        config.fused_mlp = True
        config.fused_dropout_add_ln = True

    # if not rotary, we load the weight from HF but ignore the position embeddings.
    # The model would be nonsense but it doesn't matter for the test.
    model = GPTLMHeadModel.from_pretrained(
        model_name, config, strict=not rotary, device=device, dtype=dtype
    )
    model.eval()

    if not rotary:
        model_ref = GPT2LMHeadModelHF.from_pretrained(model_name).to(device=device)
        model_hf = GPT2LMHeadModelHF.from_pretrained(model_name, torch_dtype=dtype).to(
            device=device
        )
        model_ref.eval()
        model_hf.eval()

    torch.manual_seed(0)
    tokenizer = GPT2Tokenizer.from_pretrained("gpt2")
    input_ids = tokenizer("Hello, my dog is cute and he", return_tensors="pt").input_ids.to(
        device=device
    )
    max_length = 25
    # input_ids = torch.randint(0, 100, (2, 10), dtype=torch.long, device='cuda')
    # max_length = input_ids.shape[1] + 40

    # Slow generation for reference
    sequences = []
    scores = []
    cur_input_ids = input_ids
    with torch.inference_mode():
        scores.append(model(cur_input_ids).logits[:, -1])
        sequences.append(scores[-1].argmax(dim=-1))
        for _ in range(input_ids.shape[1] + 1, max_length):
            cur_input_ids = torch.cat([cur_input_ids, rearrange(sequences[-1], "b -> b 1")], dim=-1)
            scores.append(model(cur_input_ids).logits[:, -1])
            sequences.append(scores[-1].argmax(dim=-1))
    sequences = torch.cat([input_ids, torch.stack(sequences, dim=1)], dim=1)
    scores = tuple(scores)

    out = model.generate(
        input_ids=input_ids,
        max_length=max_length,
        return_dict_in_generate=True,
        output_scores=True,
        enable_timing=True,
    )
    print(out.sequences)
    print(tokenizer.batch_decode(out.sequences.tolist()))
    if getattr(config, "use_flash_attn", False):
        out_cg = model.generate(
            input_ids=input_ids,
            max_length=max_length,
            cg=True,
            return_dict_in_generate=True,
            output_scores=True,
            enable_timing=True,
        )
        print(out_cg.sequences)
        assert torch.equal(torch.stack(out.scores, dim=1), torch.stack(out_cg.scores, dim=1))

    if not rotary:
        out_hf = model_hf.generate(
            input_ids=input_ids,
            max_length=max_length,
            return_dict_in_generate=True,
            output_scores=True,
        )
        out_ref = model_ref.generate(
            input_ids=input_ids,
            max_length=max_length,
            return_dict_in_generate=True,
            output_scores=True,
        )

        print(
            f"Scores max diff: {(torch.stack(out.scores, 1) - torch.stack(out_ref.scores, 1)).abs().max().item()}"
        )
        print(
            f"Scores mean diff: {(torch.stack(out.scores, 1) - torch.stack(out_ref.scores, 1)).abs().mean().item()}"
        )
        print(
            f"HF fp16 max diff: {(torch.stack(out_hf.scores, 1) - torch.stack(out_ref.scores, 1)).abs().max().item()}"
        )
        print(
            f"HF fp16 mean diff: {(torch.stack(out_hf.scores, 1) - torch.stack(out_ref.scores, 1)).abs().mean().item()}"
        )
        print(tokenizer.batch_decode(out_ref.sequences.tolist()))

    assert torch.all(out.sequences == sequences)
    assert torch.allclose(
        torch.stack(out.scores, dim=1), torch.stack(scores, dim=1), rtol=rtol, atol=atol
    )
    if not rotary:
        assert torch.all(out.sequences == out_ref.sequences)
        assert torch.all(out.sequences == out_hf.sequences)

        assert (
            torch.stack(out.scores, 1) - torch.stack(out_ref.scores, 1)
        ).abs().max().item() < 3 * (
            torch.stack(out_hf.scores, 1) - torch.stack(out_ref.scores, 1)
        ).abs().max().item()
Check that our implementation of GPT2 generation matches the HF implementation:
    the scores in fp16 should be around the same as the HF scores in fp16, when compared to
    the HF scores in fp32.
    
tests/models/test_gpt.py