[DDP][Grad compression] Fix fp16 cpp hook (#63375)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63375
I think tensor.copy_(tensor.to(torch::kFloat16)); will keep it as
float32.
Tested by add the following line:
```
LOG(INFO) << "Type is: " << compressed_tensor.scalar_type();
```
before:
```
I0816 17:03:09.823688 364141 default_comm_hooks.cpp:21] Type is: Float
```
after:
```
I0816 17:01:16.779052 353924 default_comm_hooks.cpp:21] Type is: Half
```
ghstack-source-id: 136056092
Test Plan: ci
Reviewed By: SciPioneer
Differential Revision: D30356256
fbshipit-source-id: 8208a705acd7628541cd43c8bf61d007dfdd2435
