[TVMC] Fix to check whether a path passed to --target is strictly a f…

…ile (apache#7663)

* When we use file with --target, the validation in place was only
   checking whether it was a valid path. For the case in which the
   path is a directory, it causes a crash when tvmc then tries to
   open the path.
 * This fix moved the check to be strictly for files, not only a valid
   path

python/tvm/driver/tvmc/common.py

@@ -280,7 +280,7 @@ def target_from_cli(target):
     """
     extra_targets = []
 
-    if os.path.exists(target):
+    if os.path.isfile(target):
         with open(target) as target_file:
             logger.debug("target input is a path: %s", target)
             target = "".join(target_file.readlines())