Refactor data type handling to use mscclpp::DataType enum

[Copilot]

Replaces int and ncclDataType_t parameters with strongly-typed mscclpp::DataType enum throughout the API to improve type safety and reduce ambiguity.

Changes

• Core API: Define DataType enum in gpu_data_types.hpp with standard GPU data types (Int8, Uint8, Int32, Uint32, Int64, Uint64, Float16, Float32, BFloat16)
• Algorithm interfaces: Update all algorithm APIs (AllReduce, AllGather, AllToAll, ReduceScatter) to accept DataType instead of int
• NCCL interop: Add ncclDataTypeToMscclpp() conversion in apps/nccl/src/datatype_conversion.hpp with getDataTypeSize() helper
• GPU kernels: Add mscclppDataType type alias in gpu.hpp for device-side usage

Example

// Before
AllReduce allreduce(/* ... */, int dtype);

// After  
AllReduce allreduce(/* ... */, mscclpp::DataType dtype);

// NCCL conversion
auto msccl_dtype = ncclDataTypeToMscclpp(nccl_dtype);
size_t elem_size = getDataTypeSize(msccl_dtype);

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