enable padding with custom value, default to 0

[mtin]

for one of our use cases when limiting weights to -1 and +1 we want to be able to pad with -1 instead of 0 - not sure if this is helpful for anybody else, thought I'd check with you via this PR!

[szha]

This code base has been donated to the Apache MXNet project per #373, and repo is deprecated. Future development should continue in Apache MXNet.

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