Added test coverage for batched_parmap

[atapin]

Description

Added test coverage for batched_parmap function and improved its readability by simplifying the implementation. Also improved the performance by utilizing list.extend method instead of + concatenation. Extension modifies a list in place, instead of creating a new list by +

Motivation and Context

Currently batched_parmap has unnecessary cyclomatic complexity, because it has to check for trailing chunks of batches in case when size of args is not divisible by batch_size. Even though the check is implemented, this case is not covered by tests.
Also, memory consumption of this function can be reduced by replacing list concatenation with list extension.

Fixes #104

How has this been tested?

I provided a unit test and ran all other tests.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.