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/algorithm.cpp

@@ -75,15 +75,17 @@ void register_algorithm(nb::module_& m) {
               [](Algorithm& self, std::shared_ptr<Communicator> comm, uintptr_t input, uintptr_t output,
                  size_t inputSize, size_t outputSize, DataType dtype, ReduceOp op, uintptr_t stream,
                  std::shared_ptr<Executor> executor, int nBlocks, int nThreadsPerBlock, bool symmetricMemory,
-                 std::unordered_map<std::string, uintptr_t> extras) {
+                 std::unordered_map<std::string, uintptr_t> extras, int32_t accumDtype) {
                 return self.execute(comm, reinterpret_cast<const void*>(input), reinterpret_cast<void*>(output),
                                     inputSize, outputSize, dtype, op, reinterpret_cast<cudaStream_t>(stream), executor,
-                                    nBlocks, nThreadsPerBlock, symmetricMemory, extras);
+                                    nBlocks, nThreadsPerBlock, symmetricMemory, extras,
+                                    static_cast<DataType>(accumDtype));
               },
               nb::arg("comm"), nb::arg("input"), nb::arg("output"), nb::arg("input_size"), nb::arg("output_size"),
               nb::arg("dtype"), nb::arg("op") = ReduceOp::NOP, nb::arg("stream") = 0, nb::arg("executor") = nullptr,
               nb::arg("n_blocks") = 0, nb::arg("n_threads_per_block") = 0, nb::arg("symmetric_memory") = false,
-              nb::arg("extras") = std::unordered_map<std::string, uintptr_t>())
+              nb::arg("extras") = std::unordered_map<std::string, uintptr_t>(),
+              nb::arg("accum_dtype") = static_cast<int32_t>(DataType::AUTO))
           .def("reset", &Algorithm::reset);
 
   nb::class_<Algorithm::Constraint>(algorithmClass, "Constraint")