Move float16 into fluid folder

[kexinzhao]

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[CLAassistant]

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[wangkuiyi]

Thanks to @kexinzhao for this PR! It looks good to me!

Just try to confirm, if fluid/platform or fluid/framework a better place than fluid/operators/math directory for FP16?

[kexinzhao]

Thanks @wangkuiyi for the suggestion!

fluid/framework seems to be a better place, since in framework folder, we have some related files like data_type.h and data_type_transform. float16 is designed to work on both CPU and GPU, so we prefer not to put it in fluid platform folder.

I will modify this pr to put FP16 in fluid/framework.

[kexinzhao]

After discussion with @wangkuiyi, we think given that float16 has implementation using CUDA and is thus platform dependent. We prefer to put it in the platform folder.