Add per_gpu_scaling = False for distributed training

[andriilitvynchuk]

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Description

If per_gpu_scaling: False and distributed mode is on, we divide batch_size by the number of GPUs so in total batch_size will stay the same. If batch_size is not divisible by the number of GPUs we raise ValueError.

Related Issue

#1023

Type of Change

• Examples / docs / tutorials / contributors update
• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves an existing feature)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

PR review

Anyone in the community is free to review the PR once the tests have passed.
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PS

• I know, that I could join slack for pull request discussion.