Fix batchnorm in testmode without track stats

In test mode, the CUDA cuDNN implementation of batchnorm was not
matching the CPU batchnorm in FLUX. In FLUX, with track_stats=False, the
mean and variance of the current batch are used. Here, mean and variance
were initialized to 0 and 1, respectively, and passed to
cudnnBatchNormalizationForwardInference.

To fix this, we need to calculate the mean and variance over the current
batch to match the CPU implementation. Unfortunately,
cudnnBatchNormalizationForwardInference requires a trained running mean
and variance. However, batchnorm train and test should be identical
without tracked stats since they both normalize over the current
batch. As a result we can use cudnnBatchNormalizationForwardTraining in
test mode as well, which works without a running mean and variance.

ext/NNlibCUDACUDNNExt/batchnorm.jl

@@ -84,7 +84,15 @@ function cudnnBNForward!(y::DenseCuArray{T}, g::DenseCuArray{T}, b::DenseCuArray
       cache.ivar = ivar
     end
   else
-    cudnnBatchNormalizationForwardInference(handle(), CUDNN_BATCHNORM_SPATIAL, scalingParameter(T, alpha), scalingParameter(T, beta), xd, x, yd, y, gd, g, b, running_mean, running_var, eps)
+    if track_stats
+      cudnnBatchNormalizationForwardInference(handle(), CUDNN_BATCHNORM_SPATIAL, scalingParameter(T, alpha), scalingParameter(T, beta), xd, x, yd, y, gd, g, b, running_mean, running_var, eps)
+    else
+      # cudnnBatchNormalizationForwardInference does not accept CV_NULL for running_mean
+      # and running_var. We could calculate mean and var of `x` here, but instead use
+      # cudnnBatchNormalizationFowardTraining. cudnnBatchNormalizationForwardTraining does
+      # accept CV_NULL and will calculate mean and var itself.
+      cudnnBatchNormalizationForwardTraining(handle(), CUDNN_BATCHNORM_SPATIAL, scalingParameter(T, alpha), scalingParameter(T, beta), xd, x, yd, y, gd, g, b, momentum, CU_NULL, CU_NULL, eps, CU_NULL, CU_NULL)
+    end
   end
   return y
 end

test/ext_cuda/batchnorm.jl

@@ -1,3 +1,5 @@
+using Statistics
+
 @testset "Batchnorm" begin
     v = CUDA.rand(Float32, 2)
     m = CUDA.rand(Float32, 2, 5)
@@ -24,4 +26,13 @@
             @test_throws ArgumentError batchnorm(v, v, m, α, β, 1.0; kws...)
         end
     end 
+    @testset "test mode" begin
+        y_no_track_stats = batchnorm(v, v, m, nothing, nothing, 1.0; training=false, track_stats=false)
+        running_mean = mean(m, dims=[2])
+        running_var = var(m, mean=running_mean, dims=[2], corrected=false)
+        y_track_stats = batchnorm(v, v, m, running_mean, running_var, 1.0; training=false, track_stats=true)
+        # batchnorm without tracked stats should equal bathnorm with tracked stats where the
+        # stats are calculated only on the input.
+        @test y_no_track_stats ≈ y_track_stats
+    end
 end