Rename MLOperandDescriptor's dimensions to shape

[a-sully]

As discussed in #666:

MLOperandDescriptor takes a dimensions field, while querying this field from an MLOperand is done with the shape() method. As per https://w3ctag.github.io/design-principles/#attribute-reuse, we should re-use terms rather than unnecessarily introducing new vocabulary.

A quick CTRL-F of the spec text shows:

• 139 instances of "dimensions", mostly due to MLOperandDescriptor.dimensions
• 483 instances of "shape", which seems to be used more commonly in prose (e.g. "the shape of the input tensor")

We should decide to use one or the other (and possibly continue this discussion on another issue regardless)

"shape" is the more widely used term, so I propose we align on that. WDYT?