Fix Zero-3 static scale assertion in fp16 test (#7866)
PR #7839 introduced a regression by changing `TestZeroStaticScale` from
`assert optim.dynamic_loss_scale == False` to `assert
optim.loss_scale_config.dynamic_loss_scale == False`.
`loss_scale_config` is not part of the ZeRO optimizer (only non-ZeRO
optimizer have it), while this test runs with ZeRO optimizers.
With this fix, `TestZeroStaticScale` now passes for stages 1/2/3.
Signed-off-by: Masahiro Tanaka <mtanaka@anyscale.com>
