Add RFFT kernel to MicroMutableOpResolver.

[shlmregev]

This change includes the RFFT kernel header, making the RFFT operation specialized for different types (int16 etc.) available for use in TFLite Micro models via the MicroMutableOpResolver.

BUG=315941833

[ddavis-2015]

@shlmregev @veblush I am confused by this PR change. Could not the application developer have included signal/micro/kernels/rfft.h in their own code, and then called AddRfft with the registration of their choice?

[shlmregev]

@shlmregev @veblush I am confused by this PR change. Could not the application developer have included signal/micro/kernels/rfft.h in their own code, and then called AddRfft with the registration of their choice?

The visibility for register_signal_ops is limited to "//tensorflow/lite/micro" and that's the only rule that exposes this header.
It look like including the header in micro_mutable_op_resolver.h is the preferred way of doing this.

[veblush]

I think the standard approach here is to add the register functions to

tflite-micro/tensorflow/lite/micro/kernels/micro_ops.h

Lines 145 to 158 in 9cbf6e0

	namespace tflm_signal {
	TFLMRegistration* Register_DELAY();
	TFLMRegistration* Register_FFT_AUTO_SCALE();
	TFLMRegistration* Register_FILTER_BANK();
	TFLMRegistration* Register_FILTER_BANK_LOG();
	TFLMRegistration* Register_FILTER_BANK_SPECTRAL_SUBTRACTION();
	TFLMRegistration* Register_FILTER_BANK_SQUARE_ROOT();
	TFLMRegistration* Register_ENERGY();
	TFLMRegistration* Register_FRAMER();
	TFLMRegistration* Register_OVERLAP_ADD();
	TFLMRegistration* Register_PCAN();
	TFLMRegistration* Register_STACKER();
	TFLMRegistration* Register_WINDOW();
	} // namespace tflm_signal

instead of including new headers.

[shlmregev]

The reason why I need to include this file, is so that I can do this:
AddRfft(::tflite::tflm_signal::Register_RFFT_INT16());

Should I add ::tflite::tflm_signal::Register_RFFT_INT16() to micro_ops.h?

micro_ops.h doesn't define any other Register_ function for specialized types. And looking at tensorflow/lite/micro/micro_mutable_op_resolver.h, there are multiple kernel headers included there, presumably for the same purpose.

[ddavis-2015]

@shlmregev @veblush Ah, I understand now. This is how CONV_2D, FULLY_CONNECTED, MUL, etc. expose their additional registrations as well. So micro_mutable_op_resolver.h is the right place to put the include. I'm thinking that signal/micro/kernels/irfft.h should also be included here, as it also has multiple registrations.