Added cross-stream grouping to batches and nireqs in optimizer

[pbartosik]

Description

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Fixes # (issue)

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[oonyshch]

in the grouping logic for both BatchGenerator and NireqGenerator
the if instance_id: checks might fail for empty string values, causing elements with the same empty instance_id to be ungrouped when they should share the same batch/nireq group.

upd: @walidbarakat corrected me right
LGTM then :)

[walidbarakat]

LGTM ...

@pbartosik a reminder please to spend few sentences to explain the decision and impact of change. that would save time and effort for reviewers

@oonyshch instance grouping happens only when user explicitly set model-instance-id to be the same value/id, otherwise - if model-instance-id is not set or set to null - OV will set automatically the instance id to be unique. in other words, each instance id is unique and each instance/stream is a group consists of one member .. unless the user specify otherwise.