simple functorch with modes

[samdow]

This adds a rewrite of the simple functorch notebook to use torch dispatch modes. In doing this, we needed to add padding to vmap (i.e. we need to make vmap(f)(torch.randn(2), torch.randn(2, 4)) work)

Still TODO:

• fix bugs identified in custom_vjp bug #31

It raised a couple bugs in torch dispatch modes

• fix bug to set ancestors if inner is explicitly given
• fix bug to make enable_torch_dispatch_mode(None, replace=True) work

[samdow]

Note that because we're sitting at the torch dispatch level, autograd has already happened so we still have to rewrite autograd here and can't reuse PyTorch's autograd. This would be a no-go in actual functorch so we'll need to figure out a solution here