Fix JIT failure on SM100+ with CUDA <= 13.0: use 64-bit size operand for st.bulk

[MengYu10151]

Summary

st_bulk() in deep_ep/include/deep_ep/common/ptx.cuh passes the st.bulk size operand in a 32-bit register ("r" constraint). The PTX ISA defines this operand as .u64 — the 32-bit form only became legal in PTX ISA 9.0 and is first implemented in ptxas 13.1 (its own diagnostic states: Feature 'st.bulk with 32-bit size argument' requires PTX ISA .version 9.0 or later).

As a result, on CUDA 12.8–13.0 toolchains, runtime JIT compilation of dispatch_copy_epilogue fails on SM100-family GPUs whenever dispatch(..., do_expand=True, do_zero_padding=True) is used. This is not a rare parameter combination: the standard test suite tests/elastic/test_ep.py exercises it in its default flow, so on a clean SM100-family environment (no JIT cache) with CUDA <= 13.0 the first test run fails:

ptxas /tmp/xxx.ptx, line 401; error   : Arguments mismatch for instruction 'mov'
ptxas fatal   : Ptx assembly aborted due to errors

This is likely invisible in environments already on CUDA 13.1+.

Changes

One line: pass the size as a 64-bit operand ("l" constraint), which is accepted by every toolkit version.

Verification

Verified on 8x B300 (sm_103) with tests/elastic/test_ep.py --test-first-only (EP_DISABLE_GIN=1), and by directly compiling the JIT-generated dispatch_copy_epilogue_impl<true, true, true, ...> kernel across toolkits:

ptxas version	32-bit "r" (before)	64-bit "l" (after)
CUDA 12.9	❌ Arguments mismatch for instruction 'st.bulk'	✅ compiles
CUDA 13.0	❌ Arguments mismatch for instruction 'mov' — runtime JIT failure on all ranks, test aborts	✅ compiles, full test passes
CUDA 13.1	✅ compiles	✅ compiles

No behavior or performance change where both forms compile: with CUDA 13.1 the generated SASS for the affected kernel is byte-identical before/after (cuobjdump -sass diff is empty, identical register/shared-memory usage), and an 8-GPU A/B run shows identical dispatch bandwidth and passing correctness checks.

To pin down the exact trigger condition, all 8 <kDoExpand, kCachedMode, kDoZeroPadding> template combinations of dispatch_copy_epilogue_impl were compiled with CUDA 13.0 against the unfixed header: only the two instantiations with kDoExpand && kDoZeroPadding (<true, false, true> and <true, true, true>) fail; every other combination compiles fine, since the st_bulk() call sits behind if constexpr (kDoZeroPadding and kDoExpand).

[alpha-baby]

LGTM

[ds-review-bot]

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