[NumPy] Fix uses of the np.stack() family of functions under NumPy 1.25.

NumPy 1.25 release notes: https://github.com/numpy/numpy/releases/tag/v1.25.0

Per the release notes, in NumPy 1.25:

A sequence must now be passed into the stacking family of functions (stack, vstack, hstack, dstack and column_stack). (gh-23019)

This change fixes test failures where a non-sequence was passed to a stack function. The most common fix is to convert the input to a list explicitly before passing it to any stacking functions.

PiperOrigin-RevId: 547165426

magenta/models/music_vae/data_test.py

@@ -984,8 +984,11 @@ def testSliced(self):
     tensors = converter.to_tensors(self.sequence)
     self.assertArraySetsEqual(tensors.inputs, tensors.outputs)
     actual_sliced_labels = [
-        np.stack(np.argmax(s, axis=-1) for s in np.split(t, [90, 180], axis=-1))
-        for t in tensors.outputs]
+        np.stack(
+            [np.argmax(s, axis=-1) for s in np.split(t, [90, 180], axis=-1)]
+        )
+        for t in tensors.outputs
+    ]
 
     self.assertArraySetsEqual(self.expected_sliced_labels, actual_sliced_labels)
 
@@ -999,8 +1002,11 @@ def testSlicedChordConditioned(self):
     tensors = converter.to_tensors(self.sequence)
     self.assertArraySetsEqual(tensors.inputs, tensors.outputs)
     actual_sliced_labels = [
-        np.stack(np.argmax(s, axis=-1) for s in np.split(t, [90, 180], axis=-1))
-        for t in tensors.outputs]
+        np.stack(
+            [np.argmax(s, axis=-1) for s in np.split(t, [90, 180], axis=-1)]
+        )
+        for t in tensors.outputs
+    ]
     actual_sliced_chord_labels = [
         np.argmax(t, axis=-1) for t in tensors.controls]
 
@@ -1018,8 +1024,11 @@ def testSlicedKeyConditioned(self):
     tensors = converter.to_tensors(self.sequence)
     self.assertArraySetsEqual(tensors.inputs, tensors.outputs)
     actual_sliced_labels = [
-        np.stack(np.argmax(s, axis=-1) for s in np.split(t, [90, 180], axis=-1))
-        for t in tensors.outputs]
+        np.stack(
+            [np.argmax(s, axis=-1) for s in np.split(t, [90, 180], axis=-1)]
+        )
+        for t in tensors.outputs
+    ]
     actual_sliced_key_labels = [
         np.argmax(t, axis=-1) for t in tensors.controls]
 
@@ -1038,8 +1047,11 @@ def testSlicedChordAndKeyConditioned(self):
     tensors = converter.to_tensors(self.sequence)
     self.assertArraySetsEqual(tensors.inputs, tensors.outputs)
     actual_sliced_labels = [
-        np.stack(np.argmax(s, axis=-1) for s in np.split(t, [90, 180], axis=-1))
-        for t in tensors.outputs]
+        np.stack(
+            [np.argmax(s, axis=-1) for s in np.split(t, [90, 180], axis=-1)]
+        )
+        for t in tensors.outputs
+    ]
     actual_sliced_chord_labels = [
         np.argmax(t[:, :-12], axis=-1) for t in tensors.controls]
     actual_sliced_key_labels = [