[GitHub] [incubator-tvm] masahi opened a new pull request #4888: improve antlr import error message

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masahi opened a new pull request #4888: improve antlr import error message
URL: https://github.com/apache/incubator-tvm/pull/4888
 
 
   Currently, the error message I get from the parser import failure is 
confusing.
   
   If I try to use VM functionality, I get an error `Couldn't find ANTLR 
parser. Try building with USE_ANTLR=ON.` But even if I rebuild TVM with 
USE_ANTLR=ON, I still get the same error because the generated parser try to 
import the antlr4 python package that I haven't installed, but the error still 
says `Try building with USE_ANTLR=ON`. 
   
   This is confusing, I think 
   ```Python
   try:
       from antlr4 import InputStream, CommonTokenStream
       from antlr4.error.ErrorListener import ErrorListener
   except ImportError:
       raise Exception("Couldn't find ANTLR runtime." +
                       "Try running `pip{version} install 
antlr4-python{version}-runtime`."
                       .format(version=PYTHON_VERSION))
   ```
   
   should come before
   
   ```Python
   try:
       from .grammar.py3.RelayVisitor import RelayVisitor
       from .grammar.py3.RelayParser import RelayParser
       from .grammar.py3.RelayLexer import RelayLexer
   except ImportError:
       raise Exception("Couldn't find ANTLR parser. Try building with 
USE_ANTLR=ON.")
   ```
   because the latter depends on the antlr4 python package.
   
   please review @wweic @zhiics @icemelon9 

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