Fix Fast Fourier Transforms in operator API

* Change ignore list in generate_api.py to generate operators
information.
* Fix serialization attributes mapping for operator FFT and RFFT
* Add a unit test for Fft2d and Rfft2d operator

Change-Id: I3ad7a77a3c46aa586834188bab42cbdcc423e834
Signed-off-by: Dhruv Chauhan <dhruv.chauhan@arm.com>
diff --git a/scripts/operator_api/generate_api.py b/scripts/operator_api/generate_api.py
index d9077f0..7f10568 100644
--- a/scripts/operator_api/generate_api.py
+++ b/scripts/operator_api/generate_api.py
@@ -62,6 +62,8 @@
         "conv3d": "Conv",
         "depthwise_conv2d": "Conv",
         "fully_connected": "FullyConnected",
+        "fft2d": "FFT",
+        "rfft2d": "RFFT",
         "matmul": "MatMul",
         "max_pool2d": "Pool",
         "transpose_conv2d": "TransposeConv",
@@ -236,8 +238,6 @@
         "cond_if",
         "const",
         "custom",
-        "fft2d",
-        "rfft2d",
         "variable",
         "variable_read",
         "variable_write",