[MetalPerformanceShaders] Make MPSCnnConvolutionDataSource.Load a method in .NET. Fixes #6530. (#14087)

That's also how Swift defines it.

Fixes https://github.com/xamarin/xamarin-macios/issues/6530.

src/metalperformanceshaders.cs

@@ -5541,7 +5541,11 @@ namespace MetalPerformanceShaders {
 
 		[Abstract]
 		[Export ("load")]
+#if NET
+		bool Load ();
+#else
 		bool Load { get; }
+#endif
 
 		[Abstract]
 		[Export ("purge")]