De-flakify and re-enable tests in replicate_model_fn_test.py.

"Reduce metric variables" operation is a single operation across all metric variables, which means it is across all eval metrics.  Previously, an update op for every eval metric was conditioned on a copy of overall "reduce metric variables" op.  The latter was meant to be idempotent and thus the end result was supposed to be correct.

However, "reduce metric variables" op consists of a number of variable assignments and thus is not atomic.  If execution of two "reduce metric variables" ops interleaves, then the end result might come out to be incorrect.  This caused flakiness in replicate_model_fn_test.py.  To fix the problem, there is now a single copy of the "reduce metric variables" and every eval metric is associated with that single instance.

PiperOrigin-RevId: 175232016