add switch for onnx exporter (#564)

mmcv/onnx/symbolic.py

 """Modified from https://github.com/pytorch/pytorch."""
+import os
+
 import numpy as np
 import torch
 from torch.nn.modules.utils import _pair, _single, _triple
@@ -21,14 +23,27 @@ def _interpolate(name, dim, interpolate_mode):
             'Constant', value_t=torch.tensor([], dtype=torch.float32))
 
         if scales is None:
-            input_size = g.op('Shape', input)
-            input_size_beg = sym_help._slice_helper(
-                g, input_size, axes=[0], ends=[2], starts=[0])
-            output_size = g.op(
-                'Cast',
-                output_size,
-                to_i=sym_help.cast_pytorch_to_onnx['Long'])
-            output_size = g.op('Concat', input_size_beg, output_size, axis_i=0)
+            if 'ONNX_BACKEND' in os.environ and os.environ[
+                    'ONNX_BACKEND'] == 'TensorRT':
+                input_size = input.type().sizes()
+                # slice the first two dim
+                input_size = input_size[:2]
+                # convert output_size to int type
+                output_size = sym_help._maybe_get_const(output_size, 'is')
+                input_size.extend(output_size)
+                output_size = g.op(
+                    'Constant',
+                    value_t=torch.tensor(input_size, dtype=torch.int64))
+            else:
+                input_size = g.op('Shape', input)
+                input_size_beg = sym_help._slice_helper(
+                    g, input_size, axes=[0], ends=[2], starts=[0])
+                output_size = g.op(
+                    'Cast',
+                    output_size,
+                    to_i=sym_help.cast_pytorch_to_onnx['Long'])
+                output_size = g.op(
+                    'Concat', input_size_beg, output_size, axis_i=0)
             scales = g.op(
                 'Constant', value_t=torch.tensor([], dtype=torch.float32))
             return g.op(