[BUG] nn.Hardswish does not exist in pytorch 1.5

[ShoufaChen]

Describe the bug
07d952c implemented the levit, which uses nn.Hardswish.

However, nn.Hardswish does not exist in pytorch1.5.

    from .models import create_model, list_models, is_model, list_modules, model_entrypoint, \
  File "/mnt/lustre/chenshoufa/workspace/deit_timm/timm/models/__init__.py", line 19, in <module>
    from .models import create_model, list_models, is_model, list_modules, model_entrypoint, \
  File "/mnt/lustre/chenshoufa/workspace/deit_timm/timm/models/__init__.py", line 19, in <module>
    from .levit import *
  File "/mnt/lustre/chenshoufa/workspace/deit_timm/timm/models/levit.py", line 428, in <module>
    from .levit import *
  File "/mnt/lustre/chenshoufa/workspace/deit_timm/timm/models/levit.py", line 428, in <module>
    class Levit(nn.Module):
  File "/mnt/lustre/chenshoufa/workspace/deit_timm/timm/models/levit.py", line 446, in Levit
    class Levit(nn.Module):
  File "/mnt/lustre/chenshoufa/workspace/deit_timm/timm/models/levit.py", line 446, in Levit
    act_layer=nn.Hardswish,
AttributeError: module 'torch.nn' has no attribute 'Hardswish'

Currently I just removed levit import because I do not need it.
I wonder if there is an elegant way to hack this bug.

[rwightman]

@ShoufaChen right, joys of python, always forget that default args are evaluated on defn. I'll fix this one, but a warning, I don't intend to keep supporting PyTorch version < 1.7 at this point. 1.4 to 1.6 should still work for majority of models, and I'll fix outright breaks of the while lib like this, but you should consider migrating...