add gradient test for transformations

fix

Author: yuehhua

test/Transform/chebyshev_transform.jl

@@ -8,4 +8,7 @@
     @test size(transform(t, 𝐱)) == (30, 40, 50, ch, batch)
     @test size(truncate_modes(t, transform(t, 𝐱))) == (3, 4, 5, ch, batch)
     @test size(inverse(t, truncate_modes(t, transform(t, 𝐱)))) == (3, 4, 5, ch, batch)
+
+    @test_broken g = gradient(x -> sum(inverse(t, truncate_modes(t, transform(t, x)))), 𝐱)
+    @test_broken size(g[1]) == (30, 40, 50, ch, batch)
 end

test/Transform/fourier_transform.jl

@@ -8,4 +8,7 @@
     @test size(transform(ft, 𝐱)) == (30, 40, 50, ch, batch)
     @test size(truncate_modes(ft, transform(ft, 𝐱))) == (3, 4, 5, ch, batch)
     @test size(inverse(ft, truncate_modes(ft, transform(ft, 𝐱)))) == (3, 4, 5, ch, batch)
+
+    g = Zygote.gradient(x -> sum(inverse(ft, truncate_modes(ft, transform(ft, x)))), 𝐱)
+    @test size(g[1]) == (30, 40, 50, ch, batch)
 end