Fixing slow and difficult to read operation with simple matmul

[ludvik-martin]

The (inp.unsqueeze(-1) * out.g.unsqueeze(1)).sum(0) is actually doing the same thing as inp.T @ out.g, but much slower. The only reason for the operation with unsqueeze() is to exercise the broadcasting. Compared with %timeit - the method with @ is nearly order of magnitude faster (on my machine)

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[ludvik-martin]

Actually, at the end of jupyter the notebook Jeremy refactores the code in the same way... Should have read it properly to the end :)