tests: Add batch size 1 cases to test_trtllm_gen_attention.py that fail, marked xfail

[bkryu]

📌 Description

Trtllm-gen's attention kernels have been discovered to fail tests when batch size is 1.

Current PR adds batch size 1 cases to:
test_trtllm_gen_prefill_deepseek: that triggers an IMA with the newly added parameters

## Running pytest ./tests/attention/test_trtllm_gen_attention.py::test_trtllm_gen_prefill_deepseek -v
>           default_generator.manual_seed(seed)
E           torch.AcceleratorError: CUDA error: an illegal memory access was encountered
E           CUDA kernel errors might be asynchronously reported at some other API call, so the stacktrace below might be incorrect.
E           For debugging consider passing CUDA_LAUNCH_BLOCKING=1
E           Compile with `TORCH_USE_CUDA_DSA` to enable device-side assertions.

/opt/conda/envs/py312/lib/python3.12/site-packages/torch/cuda/random.py:129: AcceleratorError

test_trtllm_batch_decode: that produces incorrect outputs with newly added parameters

## Running pytest ./tests/attention/test_trtllm_gen_attention.py::test_trtllm_batch_decode -v
>                   torch.testing.assert_close(
                        output.float(),
                        output_wrapper.float(),
                        rtol=1e-1,
                        atol=1e-1,
                    )
E                   AssertionError: Tensor-likes are not close!
E                   
E                   Mismatched elements: 1480 / 8192 (18.1%)
E                   Greatest absolute difference: 64.021484375 at index (0, 46, 106) (up to 0.1 allowed)
E                   Greatest relative difference: 1.625 at index (0, 56, 109) (up to 0.1 allowed)

These test cases have been marked as pytest.xfail(). To avoid a combinatorial growth of test parameter combinations, these batch size 1 cases were defined as separate test functions.

B200 status before PR: 2052 passed, 264 skipped in 177.80s (0:02:57)
B200 status after PR: 2052 passed, 264 skipped, 3 xfailed in 195.14s (0:03:15)

Status tracked in Issue 1898

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Reviewer Notes