1478 Fix TorchScript issue in AHNet with PyTorch 1.5 (#1538)

* [DLMED] fix TorchScript issue in AHNet

Signed-off-by: Nic Ma <nma@nvidia.com>

* [DLMED] add test cases

Signed-off-by: Nic Ma <nma@nvidia.com>

* [DLMED] fix torchscript issue in PyTorch 1.5

Signed-off-by: Nic Ma <nma@nvidia.com>

monai/networks/nets/ahnet.py

@@ -434,8 +434,7 @@ def __init__(
         self.dense4 = DenseBlock(spatial_dims, ndenselayer, num_init_features, densebn, densegrowth, 0.0)
         noutdense4 = num_init_features + densegrowth * ndenselayer
 
-        if psp_block_num > 0:
-            self.psp = PSP(spatial_dims, psp_block_num, noutdense4, upsample_mode)
+        self.psp = PSP(spatial_dims, psp_block_num, noutdense4, upsample_mode)
         self.final = Final(spatial_dims, psp_block_num + noutdense4, out_channels, upsample_mode)
 
         # Initialise parameters
@@ -511,7 +510,7 @@ def forward(self, x):
 
         sum4 = self.up3(d3) + conv_x
         d4 = self.dense4(sum4)
-        if self.psp_block_num > 0 and self.psp is not None:
+        if self.psp_block_num > 0:
             psp = self.psp(d4)
             x = torch.cat((psp, d4), dim=1)
         else: