[TensorPipe] Use the new multi-payload message API (#37919)
Summary:
In D21209901 TensorPipe added support for a vector of payloads inside each message, instead of a single one, so that users with multiple payloads can send them separately as they are instead of having to copy them into a new block of contiguous memory. The PyTorch agent is using the old API, which is preventing us from deleting it. This change has no effects on over-the-wire format and thus on performance.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37919
ghstack-source-id: 103572164
Test Plan:
On both workers
```
import os
import torch
import torch.distributed.rpc as rpc
os.environ["MASTER_ADDR"] = "127.0.0.1"
os.environ["MASTER_PORT"] = "8765"
```
On worker 0
```
rpc.init_rpc(name="foo", rank=0, backend=rpc.backend_registry.BackendType.TENSORPIPE, world_size=2, rpc_backend_options=rpc.TensorPipeRpcBackendOptions(worker_name_to_id={"foo": 0, "bar": 0}))
```
On worker 1
```
rpc.init_rpc(name="bar", rank=1, backend=rpc.backend_registry.BackendType.TENSORPIPE, world_size=2, rpc_backend_options=rpc.TensorPipeRpcBackendOptions(worker_name_to_id={"foo": 0, "bar": 0}))
```
On worker 0
```
In [15]: rpc.rpc_sync("bar", torch.add, args=(torch.full((2,2), 1), torch.full((2,2), 2)))
Out[15]:
tensor([[3., 3.],
[3., 3.]])
In [16]: rpc.rpc_sync("bar", torch.add, args=(1, 2))
Out[16]: 3
```
Differential Revision: D21425536
fbshipit-source-id: a0ec2be825556b39aff018a2834baf815a6d8fa5
