message: Fix convolution test for tf v2
  Summary:
  In v2, keras casts the input to the layer type, not the layer to
  the input type.  So, specify the layer type too.
  Manifest Task: 10908
  Test Plan: All tests ok
  Reviewers: #tensorflow, georgep
diff: @@ -9,6 +9,7 @@ import numpy as np
  import test_utils as tu
  from tensorflow.compiler.tests import xla_test
  +from tensorflow.python import ipu
  from tensorflow.python.platform import googletest
  from tensorflow.python.framework import constant_op
  from tensorflow.python.framework import ops
  @@ -77,13 +78,15 @@ class ConvGraphCachingTest(xla_test.XLATestCase):
  2,
  1,
  use_bias=False,
  - kernel_initializer=init_ops.ones_initializer())(x)
  + kernel_initializer=init_ops.ones_initializer(),
  + dtype=np.float32)(x)
  y = math_ops.cast(y, np.float16)
  y = layers.Conv2D(
  2,
  1,
  use_bias=False,
  - kernel_initializer=init_ops.ones_initializer())(y)
  + kernel_initializer=init_ops.ones_initializer(),
  + dtype=np.float16)(y)
  with ops.device('cpu'):
  report = gen_ipu_ops.ipu_event_trace()
  