Support tensor as batch in parallel external source

[stiepan]

Category:

New feature (non-breaking change which adds functionality)
Bug fix (non-breaking change which fixes an issue)

Description:

• Support specyfing batch as a single numpy array which leading extent is treated as a samples extent of a batch.
• Support returning numpy scalars from a callback

All those cases have already been supported by non-parallel external source.

Additional information:

Affected modules and functionalities:

Parallel external source, external source, tensfor flow plugin(?)

Key points relevant for the review:

• Does it enable something we would not like to enable?

Tests:

• Existing tests apply
• New tests added
  • Python tests
  • GTests
  • Benchmark
  • Other
• N/A

Checklist

Documentation

• Existing documentation applies
• Documentation updated
  • Docstring
  • Doxygen
  • RST
  • Jupyter
  • Other
• N/A

DALI team only

Requirements

• Implements new requirements
• Affects existing requirements
• N/A

REQ IDs: N/A

JIRA TASK: DALI-2979

[stiepan]

Keeping it as a draft to check first how it affects tf plugin.