Update code for mean's dtype after code review

Miscellaneous documentation updates

Remove duplicated code.

This seems like the result of a bad merge.

Fixed crash with Composite support code and GPU

The problem was that when a Composite Op had a Composite subnode, having
the subnode return an empty support code would cause the parent node to
return '\n' as support code. As a result, GPU code generation would
crash due to non-empty support code.

Thanks to Fred for writing the associated test.

Make a machanism to don't have Alloc and GpuAlloc constant folded in som...

pep8 fix + added verbose exception

Small fixes

Add shape inference for sparse/basic.py Ops

A few PEP8 fixes, string formatting and cosmetic stuff.

more pep8 fixes

various other pep8 fixes so that being pep8 compliant doesn't take as
much time

Some more typo fixes & co

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