Restore the gtag JavaScript function to re-enable metrics

[carljparker]

The associated issue is in the pytorch/tutorials repository:

Issue #2696 · [BUG]
No longer collecting star-rating data...since transition to GA4 pytorch/tutorials#2696

The gtag function is used by metrics such as star-rating in the tutorials repository (which leverages pytorch_sphinx_theme).

The definition for the gtag function was deleted in the transition to Google Analytics 4 (GA4) in PR#182, which in-turn broke our metrics.

This commit restores the definition of the function to re-enable these metrics.

[svekars]

I do not have a way to test this but approving to unblock. Make sure to verify that this is indeed working.

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