Grow DynamicScale after growth_interval finite steps, not one more

[vineethsaivs]

DynamicScale grows the loss scale on the (growth_interval + 1)th finite step in a row, not after growth_interval finite steps as its docstring says. With growth_interval=1 the scale grows every other step instead of every step.

Cause: grow checks fin_steps == growth_interval, but fin_steps is the count before the current step, so it only reaches growth_interval one step later.

Fix: count the current step, fin_steps + 1 == growth_interval. This matches torch's GradScaler, which grows on the growth_intervalth successful step.

Test: new tests/dynamic_scale_test.py. With growth_interval=3 and scale=4, six finite steps now give 4, 4, 8, 8, 8, 16 (before: 4, 4, 4, 8, 8, 8), and a non-finite step still backs off and restarts the count. Both tests fail on main and pass here.