Enable Changing the # of shards for CW resharding (#3188)

Summary:
Pull Request resolved: #3188

Currently Dynamic Sharding assumes the # of shards per embedding table stays the same:
- https://www.internalfb.com/code/fbsource/[6d270632037a1e8bca7f63500dd07fd0b213e572]/fbcode/torchrec/distributed/sharding/dynamic_sharding.py?lines=140

E.g.
- `table_0` originally sharded on ranks: [0, 1]
- Reshard API currently supports moving `table_0` shards to ranks [1, 2].
    - Where `the shard` on rank 0 will move to rank 1, and the shard on rank 1 will move to rank 2

We want to support changing the # of shards:
- e.g. table_0 originally on ranks: [0, 1] --> reshard to [0]
- Or reshard to [0, 1, 2, 3]

Here's the unit test you can modify to check if your usecase passes:
- https://www.internalfb.com/code/fbsource/[4d0d74b9f3c441e7aa35ce7102200fa0ca8c95cf]/fbcode/torchrec/distributed/tests/test_dynamic_sharding.py?lines=453-459
- Basically change the new sharding plan to be a different # of ranks  than the original sharding plan.

Note: the new total number of ranks for each embedding table should be a factor of the dimension 0 of that embedding table
- e.g. emb_table size: [4, 8], this can only be sharded on 1, 2, or 4 ranks. not 3 ranks

Reviewed By: aporialiao

Differential Revision: D78291717

fbshipit-source-id: 83ff408068d38e6f7289cc63eb85cb304ffad738

Author: isururanawaka