add GAT to CLI

[nilchia]

The GAT graph convolution type was not supported from CLI. this PR adds this to CLI

[borauyar]

@nilchia unfortunately GAT is currently not supported. That's why it was removed from the command-line.
If you try running it, you'll get this error:
Static graphs not supported in 'GATConv'

I removed it from the docstrings now to avoid confusion: see here

[nilchia]

oh, I see, thanks!