Use size_t for element counts & byte sizes

[dagardner-nv]

Description

• Continue to use TensorIndex (int32_t) as an index in tensors & dataframes.
• Use std::size_t for byte & element counts (for multi-dimensional arrays/tensors).
• Use TensorIndex for element counts for 1d tensors/arrays and things like num_rows.

Although we can only have at most 2^31 elements in any dimension, we can have 2d tensors where the total elements exceeds 2^31 as well as a byte count that exceeds 2^31.

fixes #1004

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[mdemoret-nv]

Looks good. But similar to how we have TensorIndex, can we have a TensorSize which we use in place of all the size_t? Then it will be easier to update in the future if we need.

[mdemoret-nv]

/merge