Set up PyTorch links to appear in fastai docs

from fastai.basics import *

Test links

test_eq(_mod2page(Tensor), 'tensors.html')
test_eq(_mod2page(torch.nn), 'nn.html')
test_eq(_mod2page(inspect.getmodule(nn.Conv2d)), 'nn.html')
test_eq(_mod2page(F), 'nn.functional.html')
test_eq(_mod2page(torch.optim), 'optim.html')
test_eq(_mod2page(torch.utils.data), 'data.html')

---

pytorch_doc_link

 pytorch_doc_link (name:str)

Get the URL to the documentation of a PyTorch module, class or function

	Type	Details
name	str	Name of a PyTorch module, class or function
Returns	(<class ‘str’>, None)

test_links = {
    'Tensor': 'https://pytorch.org/docs/stable/tensors.html',
    'Tensor.sqrt': 'https://pytorch.org/docs/stable/tensors.html#torch.Tensor.sqrt',
    'torch.zeros_like': 'https://pytorch.org/docs/stable/torch.html#torch.zeros_like',
    'nn.Module': 'https://pytorch.org/docs/stable/nn.html#torch.nn.Module',
    'nn.Linear': 'https://pytorch.org/docs/stable/nn.html#torch.nn.Linear',
    'F.cross_entropy': 'https://pytorch.org/docs/stable/nn.functional.html#torch.nn.functional.cross_entropy'
}
for f,l in test_links.items(): test_eq(pytorch_doc_link(f), l)