Add indexing parameter to meshgrid call

[MvWouden]

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Using a newer version or PyTorch (required for modern GPUs), the following user warning will be thrown during the training process:

 UserWarning: torch.meshgrid: in an upcoming release, it will be required to pass the indexing argument.
(Triggered internally at ..\aten\src\ATen\native\TensorShape.cpp:3191.)

Avoid this by adding the indexing parameter to the torch.meshgrid call.

See the PyTorch documentation on meshgrid for more info.