[dtensor][1/N] add torchrec even row-wise sharding example (#120260)
**Summary**
our goal is to demonstrate that DTensor's capability to represent TorchRec's parameter sharding. Currently this is done with `ShardedTensor` and theoretically `DTensor` can replace it with minor change.
This PR serves as a start of this effort by adding an example test that represents TorchRec's `ShardingType.ROW_WISE` using DTensor. Note that this PR only covers the even sharding case.
**Test Run**
`torchrun --standalone --nnodes=1 --nproc-per-node=4 torch/distributed/_tensor/examples/torchrec_sharding_example.py -e row-wise`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120260
Approved by: https://github.com/wanchaol
ghstack dependencies: #121179
