fix(aten::flatten): Fixes dynamic shape for flatten

[narendasan]

Description

Fixes flatten to work with dynamic input. Also will now error out on dynamic input and resize (issue to be filed for a shape tensor runtime evaluation system)

Type of change

Please delete options that are not relevant and/or add your own.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas and hacks
• I have made corresponding changes to the documentation and have regenerated the documentation (make html in docsrc)
• I have added tests to verify my fix or my feature
• New and existing unit tests pass locally with my changes