add PARROTS_JIT_OPTION (#816)

* code update

* add PARROTS_JIT_OPTION

* rm flow_warp_module

* fix for lint

* small fix

mmcv/utils/parrots_jit.py

+import os
+
 from .parrots_wrapper import TORCH_VERSION
 
-if TORCH_VERSION == 'parrots':
+parrots_jit_option = os.getenv('PARROTS_JIT_OPTION')
+
+if TORCH_VERSION == 'parrots' and parrots_jit_option == 'ON':
     from parrots.jit import pat as jit
 else: