TF Nightly breaks unit tests test_horovod_syncbn_gpu() and test_horovod_syncbn_cpu()

[chongxiaoc]

Bug report:

Nightly config CI:
https://github.com/horovod/horovod/runs/5327552707?check_suite_focus=true

Failure stack:

[0]<stdout>:=================================== FAILURES ===================================
[0]<stdout>:___________________ TensorFlowTests.test_horovod_syncbn_cpu ____________________
[0]<stdout>:
[0]<stdout>:self = <test_tensorflow.TensorFlowTests testMethod=test_horovod_syncbn_cpu>
[0]<stdout>:
[0]<stdout>:    def test_horovod_syncbn_cpu(self):
[0]<stdout>:        """Test that the SyncBatchNormalization implementation is correct on CPU."""
[0]<stdout>:    
[0]<stdout>:        hvd.init()
[0]<stdout>:        with tf.device("/cpu:0"):
[0]<stdout>:            x_list = [
[0]<stdout>:                tf.convert_to_tensor(np.stack([
[0]<stdout>:                    np.array([
[0]<stdout>:                        [r, r + 1],
[0]<stdout>:                        [r * 2, r * 2 + 1],
[0]<stdout>:                        [r * 3, r * 3 + 1],
[0]<stdout>:                        [r * 4, r * 4 + 1]
[0]<stdout>:                    ], dtype=np.float32)
[0]<stdout>:                    for r in range(hvd.size())
[0]<stdout>:                ]), np.float32),
[0]<stdout>:                tf.convert_to_tensor(np.stack([
[0]<stdout>:                    np.array([
[0]<stdout>:                        [r + 1],
[0]<stdout>:                        [r * 2 + 1],
[0]<stdout>:                        [r * 3 + 1],
[0]<stdout>:                        [r * 4 + 1]
[0]<stdout>:                    ], dtype=np.float32)
[0]<stdout>:                    for r in range(hvd.size())
[0]<stdout>:                ]), np.float32),
[0]<stdout>:            ]
[0]<stdout>:    
[0]<stdout>:            for x in x_list:
[0]<stdout>:                bn = tf.keras.layers.BatchNormalization(axis=1, fused=False)
[0]<stdout>:                sync_bn = hvd.SyncBatchNormalization(axis=1)
[0]<stdout>:>               bn_func = bn.apply(x, training=True)
[0]<stdout>:E               AttributeError: 'BatchNormalization' object has no attribute 'apply'

TF nightly removed apply() function:

@deprecation.deprecated( date=None, instructions=‘Please use `layer.__call__` method instead.’)
@doc_controls.do_not_doc_inheritable
def apply(self, inputs, *args, **kwargs):
“”"Deprecated, do NOT use!

We should switch to layer.__call__ for TF>=2.9.0.

Failed unit tests:
test_horovod_syncbn_gpu(): https://github.com/horovod/horovod/blob/master/test/parallel/test_tensorflow.py#L4054
test_horovod_syncbn_cpu(): https://github.com/horovod/horovod/blob/master/test/parallel/test_tensorflow.py#L4101