[BUG] Bug in NextVit reparameterize

[chuong98]

Describe the bug
When I do reparameterize the model NextViT for onnx export, it returns the error: self.norm(x). None Object is not callable.

I believe line 200 of file https://github.com/huggingface/pytorch-image-models/blob/main/timm/models/nextvit.py
should be
self.norm = nn.Identity() instead of self.norm = None

[rwightman]

@chuong98 yeah, that's wrong, but also there is an issue wrt to checking for None vs isinstance(nn.Identity()) .. need look through it more closely