[Relay][Parser] units=None unrecognized in parser

[hypercubestart]

Reproducible Code:

import tvm
from tvm import relay
v = relay.Var("a")
w = relay.Var("w")
y = relay.nn.dense(v, w, units=None)
f = relay.Function([v,w], y)
mod = tvm.IRModule()
mod['main'] = f
tvm.parser.parse(mod.astext())

Error:

error: operator `None` not found, perhaps you forgot to register it?
 --> from_string:3:26
   |
 3 |    nn.dense(%a, %w, units=None)
   |                           ^^^^

cc: @jroesch @gussmith23
also seems related to #6836 , this might be the root cause for the problem seen there