Fix default position_ids in CodeGenAttention module
Author: sgunasekar
Pull request: yes

CodeGenAttention forward throws error when position_ids are not specified.

# What does this PR do?
The current version of models.codegen.modeling_codegen.CodeGenAttention forward throws error on line 193 when position_ids are not specified and defaults to None. Added a default behavior by defining default position_ids as self.position_ids in the init.

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Fixes # (models.codegen.modeling_codegen.CodeGenAttention forward throws error on line 193 when position_ids are not specified and defaults to None)


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## Who can review?

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Comments:
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