MIONet for pytorch backend

[voloddia]

Hi Professor Lu,
Currently I see there is no MIONet class for pytorch backend.
By looking at the code for tensorflow.compat.v1, I guess I need to replace

if self.output_merger is None:
    y = torch.einsum("ip,jp->ij", y_func, y_loc)

with

if self.output_merger is None:
    y = torch.mul(y_func, y_loc)
    y = torch.sum(y, axis=1, keepdim=True)

But I'm not sure if other things need to be modified when self.output_merger is not None.
Could you please advise?