[TOPI][OP] Support Faster-RCNN Proposal OP on CPU (#4297)

* Support Proposal operator on CPU.

* PyLint space issue

* PyLint space issue

* Pylint singleton-comparison issue

tests/python/relay/test_op_level5.py

@@ -424,7 +424,7 @@ def test_proposal():
 
         func = relay.Function([cls_prob, bbox_pred, im_info], z)
         func = run_infer_type(func)
-        for target in ['cuda']:
+        for target in ['llvm', 'cuda']:
             if not tvm.module.enabled(target):
                 print("Skip test because %s is not enabled." % target)
                 continue

topi/tests/python/test_topi_vision.py

@@ -378,7 +378,7 @@ def verify_proposal(np_cls_prob, np_bbox_pred, np_im_info, np_out, attrs):
             f(tvm_cls_prob, tvm_bbox_pred, tvm_im_info, tvm_out)
             tvm.testing.assert_allclose(tvm_out.asnumpy(), np_out, rtol=1e-4)
 
-    for device in ['cuda']:
+    for device in ['llvm', 'cuda']:
         check_device(device)