Copybara import of the project:
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803cd5d4d0d5aacdc0a1b3fbc5718d32a9209554 by Cristian Garcia <cgarcia.e88@gmail.com>:
add HijaxVariable
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/flax/pull/4960 from google:hijax-variable 803cd5d4d0d5aacdc0a1b3fbc5718d32a9209554
PiperOrigin-RevId: 829599624
