Metal FFT for powers of 2 up to 2048 (#915)

* add Metal FFT for powers of 2

* skip GPU test on linux

* fix contiguity bug

* address comments

* Update mlx/backend/metal/fft.cpp

* Update mlx/backend/metal/fft.cpp

* fix bug in synch

---------

Co-authored-by: Alex Barron <abarron22@apple.com>
Co-authored-by: Awni Hannun <awni.hannun@gmail.com>
Co-authored-by: Awni Hannun <awni@apple.com>

benchmarks/python/fft_bench.py

@@ -0,0 +1,57 @@
+# Copyright © 2024 Apple Inc.
+
+import matplotlib
+import mlx.core as mx
+import numpy as np
+from time_utils import measure_runtime
+
+matplotlib.use("Agg")
+import matplotlib.pyplot as plt
+
+
+def bandwidth_gb(runtime_ms, system_size):
+    bytes_per_fft = np.dtype(np.complex64).itemsize * 2
+    bytes_per_gb = 1e9
+    ms_per_s = 1e3
+    return system_size * bytes_per_fft / runtime_ms * ms_per_s / bytes_per_gb
+
+
+def run_bench(system_size):
+    def fft(x):
+        out = mx.fft.fft(x)
+        mx.eval(out)
+        return out
+
+    bandwidths = []
+    for k in range(4, 12):
+        n = 2**k
+        x = mx.random.uniform(shape=(system_size // n, n)).astype(mx.float32)
+        x = x.astype(mx.complex64)
+        mx.eval(x)
+        runtime_ms = measure_runtime(fft, x=x)
+        bandwidths.append(bandwidth_gb(runtime_ms, system_size))
+
+    return bandwidths
+
+
+def time_fft():
+
+    with mx.stream(mx.cpu):
+        cpu_bandwidths = run_bench(system_size=int(2**22))
+
+    with mx.stream(mx.gpu):
+        gpu_bandwidths = run_bench(system_size=int(2**29))
+
+    # plot bandwidths
+    x = [2**k for k in range(4, 12)]
+    plt.scatter(x, gpu_bandwidths, color="green", label="GPU")
+    plt.scatter(x, cpu_bandwidths, color="red", label="CPU")
+    plt.title("MLX FFT Benchmark")
+    plt.xlabel("N")
+    plt.ylabel("Bandwidth (GB/s)")
+    plt.legend()
+    plt.savefig("fft_plot.png")
+
+
+if __name__ == "__main__":
+    time_fft()