benchmark for comms used in TorchRec (#3435)

Summary:
Pull Request resolved: #3435

# context
* add benchmark to `torch.distributed.all_to_all_single`.
* in this very first edition, this all_to_all_single is running with sync on the same stream.
* basic operations:
pre-comms compute (GPU compute heavy) ==> all_to_all_single comms ==> irrelevant compute (GPU compute heavy, does not depend on comms data) ==> post-comms compute (GPU compute heavy, uses the comms data)

# other changs
* extend the cmd_conf decorator so that it can support multiple-program selection in command line
```
python -m torchrec.distributed.benchmark.benchmark_comms \
  a2a_single --name=a2a_sync_base-$(git rev-parse --short HEAD || echo $USER)
```
* add a config (dataclass) class **`BenchmarkFunc`** for `benchmark_func`, which includes most common arguments used in `benchmark_func`

Reviewed By: spmex

Differential Revision: D83900855

fbshipit-source-id: 06279945cf382dbbbab8482b47c2f82976dbeef7

Author: TroyGarden

torchrec/distributed/benchmark/README.md

@@ -12,3 +12,15 @@ python -m torchrec.distributed.benchmark.benchmark_train_pipeline \
     --yaml_config=fbcode/torchrec/distributed/benchmark/yaml/sparse_data_dist_base.yml \
     --name=sparse_data_dist_base_$(git rev-parse --short HEAD || echo $USER) # overrides the yaml config
 ```
+
+## benchmark_comms usage
+- internal:
+```
+buck2 run @fbcode//mode/opt fbcode//torchrec/distributed/benchmark:benchmark_comms -- \
+    a2a_single --name=a2a_sync_base-$(hg whereami | cut -c 1-10)
+```
+- oss:
+```
+python -m torchrec.distributed.benchmark.benchmark_comms \
+  a2a_single --name=a2a_sync_base-$(git rev-parse --short HEAD || echo $USER)
+```