FeatureExtractor, FPNFeatureExpander cannot support float16

[chinakook]

FeatureExtractor, FPNFeatureExpander cannot support float16

[zhreshold]

It can support float16, but would require some knowledge of the legacy module way to cast to float16, which is a bit tricky, once I found an elegant way to do it I will update the Extractor module.

[zhreshold]

See updates here: apache/mxnet#17212