[Frontend][Relay] YOLO module is unparsable

[hypercubestart]

The module generated from the code in this tutorial: https://tvm.apache.org/docs/tutorials/frontend/from_darknet.html
causes the following error when parsed by:

import tvm
tvm.parser.parse(mod.astext())

error: expected a local variable found ., which is caused by formals (i.e %LAYERTYPE.CONVOLUTIONAL0_weight: Tensor[(32, 3, 3, 3), float32]) which the '.' character, which is unrecognized by the parser.

@jroesch recommended two options:

1. modify importer to replace invalid characters
2. modify parser to handle the '.' character