Support E4M3B15 datatype (#765)

## Summary

- **Add `fp8_e4m3b15` datatype**: A software-defined FP8 type with 4
exponent bits, 3 mantissa bits, and bias=15 (max finite value: 0.9375).
Implemented entirely in software with no HW dependency, using
Triton-style bit manipulation through fp16 as intermediate for efficient
conversion.
- **Add mixed-precision accumulation for allreduce**: All allreduce
algorithm variants (packet, NVLS packet, fullmesh, RSAG zero-copy, and
others) now support a configurable `accumDtype` parameter, enabling FP8
inputs to be reduced in float16 or float32 for higher accuracy.
- **Propagate `accumDtype` through the full API**: The new parameter is
threaded from `Algorithm::execute()` → `NativeAlgorithm` → `KernelFunc`
→ dispatch → CUDA kernels, with `DataType::AUTO` as the default
(resolves to input dtype at runtime).
- **Add FP8 accumulation correctness tests**: New `test_fp8_accum.py`
validates that higher-precision accumulation produces results at least
as accurate as native FP8 accumulation across multiple algorithms and
sizes. Skipped on CUDA SM < 89 (pre-Hopper); runs on HIP/ROCm.
- **Add `test_fp8_accum.py` to CI**: Azure Pipeline `ut.yml` now runs
FP8 accumulation tests alongside existing pytests.
- **NCCL shim logging cleanup**: Migrated `printf`-style `WARN`/`INFO`
calls to streaming-style logging.

## Key files

| Area | Files |
|------|-------|
| New datatype + vector ops | `include/mscclpp/gpu_data_types.hpp` |
| Accumulation reduce helpers | `src/core/include/reduce_kernel.hpp` |
| Algorithm API (`accumDtype`) | `include/mscclpp/algorithm.hpp`,
`src/core/algorithm.cc` |
| Allreduce kernels | `src/ext/collectives/allreduce/*.cu` |
| Dispatch + common | `src/ext/collectives/include/allreduce/common.hpp`
|
| Python bindings | `python/csrc/algorithm.cpp`,
`python/mscclpp/_core/algorithm.py` |
| Tests | `python/test/test_fp8_accum.py` |
| CI | `.azure-pipelines/templates/ut.yml` |

## Test plan

- [x] CI passes on H100 (CUDA SM 90) — full FP8 E4M3 + E4M3B15
accumulation tests
- [x] CI passes on A100 (CUDA SM 80) — FP8 tests correctly skipped
- [x] CI passes on MI300X (ROCm) — FP8 tests run via HIP
- [x] Existing `test_mscclpp.py` tests continue to pass
- [x] NCCL shim builds and runs correctly with new `accumDtype` defaults

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

python/csrc/gpu_utils_py.cpp

@@ -34,6 +34,19 @@ static DLDataType getDlType(std::string type) {
     return DLDataType{kDLBfloat, 16, 1};
   } else if (type == "torch.float16") {
     return DLDataType{kDLFloat, 16, 1};
+  } else if (type == "torch.float8_e4m3fn") {
+    return DLDataType{kDLFloat8_e4m3fn, 8, 1};
+  } else if (type == "torch.float8_e4m3fnuz") {
+    return DLDataType{kDLFloat8_e4m3fnuz, 8, 1};
+  } else if (type == "torch.float8_e5m2") {
+    return DLDataType{kDLFloat8_e5m2, 8, 1};
+  } else if (type == "torch.float8_e5m2fnuz") {
+    return DLDataType{kDLFloat8_e5m2fnuz, 8, 1};
+  } else if (type == "torch.uint8") {
+    return DLDataType{kDLUInt, 8, 1};
+  } else if (type == "fp8_e4m3b15") {
+    // No standard DLPack code for fp8_e4m3b15; store as raw uint8 bytes.
+    return DLDataType{kDLUInt, 8, 1};
   } else {
     throw Error("Unsupported type: " + type, ErrorCode::InvalidUsage);
   }