Default optimize_for_mobile() on TorchScript models (#2908)

Per https://pytorch.org/tutorials/recipes/script_optimized.html this should improve performance on torchscript models (and maybe coreml models also since coremltools operates on a torchscript model input, though this still requires testing).

models/export.py

 
 import torch
 import torch.nn as nn
+from torch.utils.mobile_optimizer import optimize_for_mobile
 
 import models
 from models.experimental import attempt_load
         print(f'\n{prefix} starting export with torch {torch.__version__}...')
         f = opt.weights.replace('.pt', '.torchscript.pt')  # filename
         ts = torch.jit.trace(model, img, strict=False)
+        ts = optimize_for_mobile(ts)  # https://pytorch.org/tutorials/recipes/script_optimized.html
         ts.save(f)
         print(f'{prefix} export success, saved as {f}')
     except Exception as e: