Fix the Op info test for digamma

1. Find the line 14 of test_ops.py and remove
   digamma from skip_list
2. Run op_info test with pytest test/test_ops.py
3. Fix the failure.

Please refer to
this guide as guide to fix:

Also refer to these PRs:

• Add bitwise_left_shift, bitwise_right_shift #7343
• Add support for baddbmm, bincount #7312
• Add support for trig, binary, algebraic ops #7325