[misc] allow overriding the TAG variable in auto_tune.sh

[kkr16]

Purpose

Allow specifying a custom TAG environment variable for auto_tune.sh.
This is useful when collecting logs from distributed jobs (e.g., on Kubernetes) where a unique identifier or timestamp needs to be coordinated externally or synced with other components, rather than relying on the script's internal execution time.
It also allows for higher resolution in the TAG, as the current TAG only offers minute precision, which can cause collisions if multiple jobs are started within the same minute.
This does NOT alter the default behaviour of auto_tune.sh.

Test Plan

1. Custom TAG: Run auto_tune.sh with a static custom tag:

TAG=custom-tag ./benchmarks/auto_tune/auto_tune.sh

2. Custom TAG + Timestamp: Run auto_tune.sh with a custom tag including a timestamp:

TAG=custom-tag-$(date +"%Y_%m_%d_%H_%M") ./benchmarks/auto_tune/auto_tune.sh

3. Default behaviour: Run auto_tune.sh without setting TAG:

./benchmarks/auto_tune/auto_tune.sh

Test Result

1. Verified that the output directory uses custom-tag.
2. Verified that the output directory uses the format custom-tag-YYYY_MM_DD_HH_MM.
3. Verified that the output directory falls back to the default timestamp format YYYY_MM_DD_HH_MM.

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[gemini-code-assist]

Code Review

This pull request correctly implements the ability to override the TAG environment variable in auto_tune.sh. However, this introduces a critical security vulnerability. The TAG variable is used to construct a directory path that is later deleted with rm -rf. Without sanitization, a malicious TAG could lead to path traversal and the deletion of arbitrary files and directories. I have added a review comment with a suggestion to sanitize the TAG variable to prevent this.