Add `ifft2` method to ops

[markveillette]

I'm curious why there is no ops.ifft2. Given that there is already fft and fft2, implementing one is trivial.

Here is an example of what an ifft2 would look like:

import keras
def keras_ops_ifft2(fft_real,fft_imag):
    """
    Inputs are real and imaginary parts of array
    of shape [...,H,W],[...,H,W] , where the last two 
    dimensions correspond tot he image dimensions

    Returns tuple containing the real and imaginary parts
    of the ifft2 

    Test:
       from keras import ops
       X = np.random.rand( 1,1,11,11 ).astype(np.float32)
       X_real,X_imag = ops.real(X), ops.imag(X)
       X_fft_real,X_fft_imag = keras.ops.fft2((X_real,X_imag))
       X_recon,_ = keras_ops_ifft2(X_fft_real,X_fft_imag)
       np.allclose(X,X_recon,atol=1e-6)
       
    """
    H = ops.cast(ops.shape(fft_real)[-2],'float32') # height
    W = ops.cast(ops.shape(fft_real)[-1],'float32') # width

    # Conjugate the input
    real_conj, imag_conj = fft_real, -fft_imag

    # Compute FFT of conjugate
    fft = ops.fft2((real_conj, imag_conj))

    return fft[0] / (H*W), -fft[1] / (H*W)

[fchollet]

We don't have 100% coverage in the ops API yet. ifft2 can definitely be added -- if you're able, please send a PR.