fix groupnorm int32 index overflow

[tlogn]

This PR fix groupnorm int32 index calculate overflow when hwc is large, as hwc is of int data type. The problem could be reproduced by code below.
@crcrpar please review, thanks !

from apex.contrib.group_norm import GroupNorm as ApexGroupNorm
import torch
layer = ApexGroupNorm(32, 128, dtype=torch.bfloat16, device='cuda', act='silu')

x = torch.randn(1, 128, 16128, 1200, dtype=torch.bfloat16, device='cuda').to(memory_format=torch.channels_last)

o = layer(x)
print(o[0][0][0][0])