Add type hints for `tensor_utils.pack()` dtype arg, and use `get_packer()`.
Add type hints for `tensor_utils.pack()` dtype arg, and use `get_packer()`.

PiperOrigin-RevId: 410000331
Change-Id: I2d6e1dae66cd7b1b09186bfc0e8b94f26fd62199

deepmind/dm_env_rpc