test: Enable testing for trtllm-gen decode bs1 (#2103)

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## 📌 Description

In #1898, it was raised that trtllm-gen's attention kernels fail for
batch size 1. The prefill kernel was fixed in #1912 and prefill tests
have been enabled.

Further updates to trtllm-gen kernels have also fixed the decode batch
size 1 issue. Current PR re-enables testing.

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needed. -->

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## 🧪 Tests

- [ ] Tests have been added or updated as needed.
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## Summary by CodeRabbit

* **Tests**
* Expanded batch_decode test scenarios to cover additional small-batch
and page-size combinations.
* Increased coverage for max_in_kv_len by testing multiple length
options instead of a single value.
* Restored previously marked-as-expected-failure case to run normally,
improving overall test pass coverage.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Zihao Ye <[email protected]>

Author: bkryu

tests/attention/test_trtllm_gen_attention.py

@@ -1041,6 +1041,7 @@ def test_trtllm_batch_decode(
     "batch_size,q_len_per_req,page_size,num_kv_heads,head_grp_size",
     [
         (1, 1, 16, 8, 8),
+        (1, 1, 32, 8, 8),
     ],
 )
 @pytest.mark.parametrize("window_left", [-1])
@@ -1052,7 +1053,7 @@ def test_trtllm_batch_decode(
 )
 @pytest.mark.parametrize("enable_pdl", [None])
 @pytest.mark.parametrize("enable_sink", [False])
-@pytest.mark.parametrize("max_in_kv_len", [8192])
+@pytest.mark.parametrize("max_in_kv_len", [4096, 8192])
 @pytest.mark.parametrize("head_dim", [128])
 @pytest.mark.parametrize("device_scale", [True, False])
 def test_trtllm_batch_decode_bs1(
@@ -1073,7 +1074,6 @@ def test_trtllm_batch_decode_bs1(
     device_scale,
 ):
     # Small number of test cases for batch size 1
-    pytest.xfail("trtllm-gen decode gets incorrect output with bs1")
     _test_trtllm_batch_decode(
         "trtllm-gen",
         kv_layout,