Add itertools.islice in validation for Dali datapipe

[oublalkhalid]

Using itertools.islice helps streamline the validation loop by limiting the number of batches processed, making validation more time-efficient without extra overhead.

PhysicsNeMo Pull Request

Description

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.
• [x] The CHANGELOG.md is up to date with these changes.
• [x] An issue is linked to this pull request.

Dependencies

[loliverhennigh]

/blossom-ci

[mnabian]

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