Return Dense Tensors for Fixed List Columns

[oliverholworthy]

Change to Output Type

Columns that have fixed shapes will return fixed size dense tensors/arrays instead of the ragged representation.

Motivation

• Load Fixed List Columns more quickly.
• Return type matches data and schema (when we have a fixed size (non-ragged) column)

By handling the special case of fixed list features where the lists are all of the same length, we can bypass some of the slower code that handles ragged lists and load these features more quickly.