Depreciate pythonic Mish and support PyTorch 1.9 version of Mish
This PR removes the old pure pythonic version of [Mish](https://arxiv.org/abs/1908.08681) and now enables support for the [PyTorch 1.9 Mish version](https://pytorch.org/docs/stable/generated/torch.nn.Mish.html#torch.nn.Mish). It also removes isolated references of the function where it is not used. 

Done