fix: Hf callback metric naming

[MikhailKardash]

Ticket

https://hpe-aiatscale.atlassian.net/browse/MD-511

Description

TRAIN = "train_progress" appears to be a confusing variable name for a value that is intended as a placeholder. This caused an issue in the behavior of on_log(). This PR renames TRAIN to DEFAULT and TRAIN_PROGRESS to TRAIN

Test Plan

Run an example with huggingface callback and make sure metrics are being recorded properly.

Checklist

• [ ] Changes have been manually QA'd
• [ ] New features have been approved by the corresponding PM
• [ ] User-facing API changes have the "User-facing API Change" label
• [ ] Release notes have been added as a separate file under docs/release-notes/ See Release Note for details.
• [ ] Licenses have been included for new code which was copied and/or modified from any external code