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:

src/context_host.cpp

@@ -122,4 +122,12 @@ __host__ void Context::barrier_all_on_stream(hipStream_t stream) {
   HOST_DISPATCH(barrier_all_on_stream(stream));
 }
 
+__host__ void Context::alltoallmem_on_stream(rocshmem_team_t team, void *dest,
+                                             const void *source, size_t size,
+                                             hipStream_t stream) {
+  ctxHostStats.incStat(NUM_HOST_ALLTOALL);
+
+  HOST_DISPATCH(alltoallmem_on_stream(team, dest, source, size, stream));
+}
+
 }  // namespace rocshmem