Start inject_hyperparams step count at 0.

[copybara-service]

Start inject_hyperparams step count at 0.

This fixes issue #415. Previously, inject_hyperparams started with
step_count=1 in the first update when using schedules (it incremented before
passing it to the schedule) whereas scale_by_schedule started with
step_count=0. To make this consistent, this PR changes inject_hyperparams
to also start at 0, i.e. increment the count only after passing it to the
schedule.

The PR comes with a test that breaks without the change. Furthermore, the step
counts in the existing tests of inject_hyperparams had to be decremented by
one in order for the tests to pass.