Add TorchScript-Lite export (WongKinYiu#387)

* TorchScript-Lite export

* forgot the import

export.py

@@ -6,6 +6,7 @@
 
 import torch
 import torch.nn as nn
+from torch.utils.mobile_optimizer import optimize_for_mobile
 
 import models
 from models.experimental import attempt_load, End2End
@@ -75,6 +76,17 @@
     except Exception as e:
         print('TorchScript export failure: %s' % e)
 
+    # TorchScript-Lite export
+    try:
+        print('\nStarting TorchScript-Lite export with torch %s...' % torch.__version__)
+        f = opt.weights.replace('.pt', '.torchscript.ptl')  # filename
+        ts = torch.jit.trace(model, img, strict=False)
+        ts = optimize_for_mobile(ts)
+        ts._save_for_lite_interpreter(f)
+        print('TorchScript-Lite export success, saved as %s' % f)
+    except Exception as e:
+        print('TorchScript-Lite export failure: %s' % e)
+
     # ONNX export
     try:
         import onnx