Reorder internal convolution and pooling operators for readability

[jon-tow]

Convolution and pooling functions were scattered around the NN.swift file along with their corresponding vjps. This PR gathers them in an organized way for good code hygiene. The only non-reordering change was a line break deletion.

[rxwei]

LGTM. Thank you!

[jon-tow]

Glad to help!

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[eaplatanios]

Note that the renaming changes I suggested will also require you to change the references to these VJP functions from other places.

[rxwei]

😕 The bad news is that it appears that one or more commits were authored or co-authored by someone other than the pull request submitter. We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that here in the pull request.

CLA doesn't work automatically with directly-committed suggestions. For formality, @eaplatanios could you please confirm that you are okay with your commits being contributed to this PR?

[eaplatanios]

Yes sure. I confirm that I am okay with my commits being contributed to this PR.

[googlebot]

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[jon-tow]

@rxwei I had to remove some "merge branch" commits that cluttered the PR's commit history just in case it bothers you all as much as it bothers me :)