Add host API for alltoallmem_on_stream collective operation (#333)

* Add host-side rocshmem_alltoallmem_on_stream function

Function signature:
  rocshmem_alltoallmem_on_stream(rocshmem_team_t team, void *dest,
                                 const void *source, size_t size,
                                 hipStream_t stream)

- The function launches rocshmem_alltoallmem_kernel which calls
device-side alltoall<char> workgroup collective through default context.
- Uses dynamic block size determination via occupancy API.
- Implemented for all backends.

* Fix incorrect sync buffer size allocation for alltoall in GDA and IPC backends

When allocating memory for alltoall_pSync_pool in setup_teams() and
teams_init() functions, the code incorrectly used ROCSHMEM_BCAST_SYNC_SIZE
instead of ROCSHMEM_ALLTOALL_SYNC_SIZE.

* Add functional test for team_alltoallmem_on_stream

This commit adds a new functional test to verify the correctness of
the host-side rocshmem_team_alltoallmem_on_stream API.

* Add documentation for rocshmem_alltoallmem_on_stream

This commit adds API documentation for the host-side
rocshmem_alltoallmem_on_stream function in the collective routines
section. The documentation includes:

[ROCm/rocshmem commit: 5577feb70d]

projects/rocshmem/src/gda/backend_gda.cpp

@@ -453,8 +453,8 @@ void GDABackend::setup_teams() {
                             * max_num_teams;
 
   alltoall_pSync_pool = reinterpret_cast<long *>(wrk_sync_pool_top_);
-  wrk_sync_pool_top_ += sizeof(long) * ROCSHMEM_BCAST_SYNC_SIZE
-                            * max_num_teams;
+  wrk_sync_pool_top_ += sizeof(long) * ROCSHMEM_ALLTOALL_SYNC_SIZE *
+                        max_num_teams;
 
   /* Accommodating for largest possible data type for pWrk */
   pWrk_pool = reinterpret_cast<void *>(wrk_sync_pool_top_);