Fix BF16_Optimizer being used without ZeRO (#7790)
The test_ds_initialize.py::TestOptimizerImplementation expects the
configuration (None, 'bf16', 'fp32') is unimplemented.
However, it is actually supported by DeepSpeed and uses FP16_Optimizer
in bf16 mode.
This PR adds the configuration in the dict of supported combinations.
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Signed-off-by: Masahiro Tanaka <mtanaka@anyscale.com>
