report an error if num_channels is not divisible by num_groups for nn.GroupNorm
For a GroupNorm module, if num_channels is not divisible by num_groups, we need to report an error when defining a module other than at the running step.
example:
```
import torch
m = torch.nn.GroupNorm(5, 6)
x = torch.randn(1, 6, 4, 4)
y = m(x)
```
before:
```
Traceback (most recent call last):
File "group_norm_test.py", line 8, in <module>
y = m(x)
File "/home/xiaobinz/miniconda3/envs/pytorch_mater/lib/python3.7/site-packages/torch/nn/modules/module.py", line 1111, in _call_impl
return forward_call(*input, **kwargs)
File "/home/xiaobinz/miniconda3/envs/pytorch_mater/lib/python3.7/site-packages/torch/nn/modules/normalization.py", line 271, in forward
input, self.num_groups, self.weight, self.bias, self.eps)
File "/home/xiaobinz/miniconda3/envs/pytorch_mater/lib/python3.7/site-packages/torch/nn/functional.py", line 2500, in group_norm
return torch.group_norm(input, num_groups, weight, bias, eps, torch.backends.cudnn.enabled)
RuntimeError: Expected number of channels in input to be divisible by num_groups, but got input of shape [1, 6, 4, 4] and num_groups=5
```
after:
```
Traceback (most recent call last):
File "group_norm_test.py", line 6, in <module>
m = torch.nn.GroupNorm(5, 6)
File "/home/xiaobinz/miniconda3/envs/pytorch_test/lib/python3.7/site-packages/torch/nn/modules/normalization.py", line 251, in __init__
raise ValueError('num_channels must be divisible by num_groups')
```
This PR also update the doc of num_groups.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74293
Approved by: https://github.com/jbschlosser
