Fix torchscript issue in ConvTranspose2d

[fmassa]

Tests have been failing recently with

RuntimeError:  Tried to access nonexistent attribute or method 'shape' of type 'Optional[Tensor]'.:   File "/opt/conda/conda-bld/torchvision_1582614418949/_test_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placeh/lib/python3.5/site-packages/torchvision/ops/misc.py", line 45             )         ]         output_shape = [x.shape[0], self.bias.shape[0]] + output_shape                                     ~~~~~~~~~~~~~~~ <--- HERE         return _new_empty_tensor(x, output_shape)

most probably due to some changes in torchscript.

This fixes it.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@5306635). Click here to learn what that means.
The diff coverage is 0%.

@@           Coverage Diff            @@
##             master   #1917   +/-   ##
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  Coverage          ?   0.43%           
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  Files             ?      92           
  Lines             ?    7395           
  Branches          ?    1113           
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  Hits              ?      32           
  Misses            ?    7355           
  Partials          ?       8

Impacted Files	Coverage Δ
torchvision/ops/misc.py	0% <0%> (ø)

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