Return a Status instead of throw an exception in GetAttrs

[snnn]

Description:

The return type of GetAttrs is Status, so it is better to return a Status instead of throwing an exception when errors happen.

It's very tricky that if you want to read a tensor shape from attrs, you can either use a std::vector or TensorShapeVector to store the data. However, if you use TensorShapeVector you must pre-compute the number of dimension before calling GetAttrs and resize the vector to the right size. std::vector doesn't have the restriction.

Motivation and Context

• Why is this change required? What problem does it solve?

Make error message more readable.

• If it fixes an open issue, please link to the issue here.

[yufenglee]