【UnitTestFix No.8】fix test_mean_op.py (#75457)

* fix TestMeanOp_Complex64ZeroSize

* v2: Increased the number of input elements in TestMeanOp_ImagNanInput

* v3: Fix gradient check for MeanOp with NaN inputs in ImagNanInput, RealNanInput, RealValuedNanInput, ZeroSize

* v4: Removed all  related content

* v5: Remove all mean_all and fix RealValuedNanInput

* v6: Fixed reduce error and mean not supporting int type error

* v7: Remove redundant int types

Author: WanRui37