MMD docstring inconsistent with implementation

[kevinchern]

The docstring shows a 1/2 factor https://github.com/dwavesystems/dwave-pytorch-plugin/blob/main/dwave/plugins/torch/nn/modules/kernels.py#L86 but the implementation absorbs that in the bandwidth https://github.com/dwavesystems/dwave-pytorch-plugin/blob/main/dwave/plugins/torch/nn/modules/kernels.py#L151