Import forward_halo_exchange to __init__.py

[jvwilliams23]

Currently forward_halo_exchange is automatically triggered when we apply a convolution to a DCTensor. In our case, we are then computing gradients of the output layer with finite differences, which we would like to do without redistributing the tensor. We therefore manually call forward_halo_exchange before this like so:

y = F.conv2d(x)
y = forward_halo_exchange(y, ...)
y = compute_spatial_derivatives(y, deltax, deltay)

so we need this function to be importable.