refactor create local process group code for distributed training in "detectron2/utils/comm.py"

## 🚀 Feature
In "detectron2/utils/comm.py", there is a function that name is "create_local_process_group".
The funciton is bellow:
```python
@functools.lru_cache()
def create_local_process_group(num_workers_per_machine: int) -> None:
"""
Create a process group that contains ranks within the same machine.

Detectron2's launch() in engine/launch.py will call this function. If you start
workers without launch(), you'll have to also call this. Otherwise utilities
like `get_local_rank()` will not work.

This function contains a barrier. All processes must call it together.

Args:
num_workers_per_machine: the number of worker processes per machine. Typically
the number of GPUs.
"""
global _LOCAL_PROCESS_GROUP
assert _LOCAL_PROCESS_GROUP is None
assert get_world_size() % num_workers_per_machine == 0
num_machines = get_world_size() // num_workers_per_machine
machine_rank = get_rank() // num_workers_per_machine
for i in range(num_machines):
ranks_on_i = list(range(i * num_workers_per_machine, (i + 1) * num_workers_per_machine))
pg = dist.new_group(ranks_on_i)
if i == machine_rank:
_LOCAL_PROCESS_GROUP = pg
```
I think the function is create dist.new_group, and set LOCAL_PROGRESS_GROUP according to machine_rank.
So they create some useless pg that i is not equal machine_rank

## Motivation & Examples
I think the function may need refactor like bellow:
```python
@functools.lru_cache()
def create_local_process_group(num_workers_per_machine: int) -> None:
global _LOCAL_PROCESS_GROUP
assert _LOCAL_PROCESS_GROUP is None
assert get_world_size() % num_workers_per_machine == 0
num_machines = get_world_size() // num_workers_per_machine
machine_rank = get_rank() // num_workers_per_machine
for i in range(num_machines):
if i == machine_rank:
ranks_on_i = list(range(i * num_workers_per_machine, (i + 1) * num_workers_per_machine))
pg = dist.new_group(ranks_on_i)
_LOCAL_PROCESS_GROUP = pg
```