[Bugfix] Clamp block table indices in align mode to prevent OOB gather

[shernshiou]

In mamba_cache_mode='align' with speculative decoding, sequences with seq_len near max_model_len caused indices_to_gather to exceed the block table column count, triggering a CUDA device-side assert in torch.gather.

Fixes #42084

Purpose

Fix an out-of-bounds torch.gather in mamba_get_block_table_tensor when
mamba_cache_mode='align' is used together with speculative decoding and a
large max_model_len.

In align mode the Mamba block table has shape
(#requests, cdiv(max_model_len, block_size)) — one column per block over
the full context window, with no extra speculative columns appended.
When computing the gather indices:

start_indices = (seq_lens - 1) // kv_cache_spec.block_size
offsets = torch.arange(1 + kv_cache_spec.num_speculative_blocks, ...)
indices_to_gather = (start_indices.unsqueeze(1) + offsets).to(torch.int64)

for a sequence with seq_len near max_model_len, start_indices is at or
near the last valid column. Adding num_speculative_blocks (which is
num_speculative_tokens when using a draft model) then pushes
indices_to_gather past the end of the block table, triggering:

torch.AcceleratorError: CUDA error: device-side assert triggered

This crashes the entire EngineCore process, killing all in-flight requests.

Fix: add a single upper-bound clamp on indices_to_gather before the
gather call, mirroring the identical .clip(max=block_table.shape[1] - 1)
pattern already used in make_local_attention_virtual_batches (line 452 in
the same file):

indices_to_gather.clamp_(max=block_table.shape[1] - 1)

Fixes #42084

Test Plan

New CPU-only regression test added to
tests/v1/attention/test_mamba_update_block_table.py:

pytest tests/v1/attention/test_mamba_update_block_table.py::test_mamba_get_block_table_tensor_align_mode_oob_clamp -v

The test constructs a block table sized for align mode
(cdiv(max_model_len, block_size) columns, no speculative tail), then calls
mamba_get_block_table_tensor with seq_lens near max_model_len and
num_speculative_blocks=2. Without the fix the test raises an index error;
with the fix it passes and verifies the output shape and value bounds.

Test Result

Before fix:

IndexError: index out of bounds

After fix:

PASSED tests/v1/attention/test_mamba_update_block_table.py::test_mamba_get_block_table_tensor_align_mode_oob_clamp

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[claude]

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[mgoin]

I don’t think this is ready to merge. The test creates a 16 column block table, but runtime allocates 18 columns here by including num_speculative_blocks, so it doesn’t reproduce a reachable state. Clamping also aliases speculative state slots, potentially turning an invariant failure into silent incorrect results. Please reproduce the actual runtime path and fix the producer