Add helper for creating MLOperandDescriptor to simplify steps (#737)

* Add helper for creating MLOperandDescriptor to simplify steps

The only non-trivial bits are a few cases where in the previous steps
the shape was calculated after the MLOperandDescriptor was created,
but this would not be an observable change to script.

* whitespace change to trigger build

* unrelated fix to trigger action

* Broadcast condition too

Co-authored-by: Ningxin Hu <[email protected]>

* Use "set" to update

Also, retain usage of "bidirectionally broadcastable"

* Update index.bs

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Co-authored-by: Ningxin Hu <[email protected]>
Co-authored-by: Dwayne Robinson <[email protected]>