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/rocshmem.cpp

@@ -999,6 +999,15 @@ __host__ void rocshmem_barrier_all_on_stream(hipStream_t stream) {
   get_internal_ctx(ROCSHMEM_HOST_CTX_DEFAULT)->barrier_all_on_stream(stream);
 }
 
+__host__ void rocshmem_alltoallmem_on_stream(rocshmem_team_t team, void *dest,
+                                             const void *source, size_t size,
+                                             hipStream_t stream) {
+  DPRINTF("Host function: rocshmem_alltoallmem_on_stream\n");
+
+  get_internal_ctx(ROCSHMEM_HOST_CTX_DEFAULT)
+      ->alltoallmem_on_stream(team, dest, source, size, stream);
+}
+
 __host__ void rocshmem_sync_all() {
   DPRINTF("Host function: rocshmem_sync_all\n");