[Relay] IR Text Format Improvements

[joshpoll]

• Sophisticated tensor and function type annotations including type relations and generics

• Easy parser installation. Currently running make will build the parser provided you have ANTLR installed; however, we'd like to make it easy for others to use the text format parser even if they don't have ANTLR installed. One way we could accomplish this is to maintain a version of the parser somewhere and have users wget it.

• Support temp vars like %0, %1, etc. We need to agree on how to properly handle these.

• Graph construction support.

• Consistency between text format and text printer. We'd like pretty_print(parse(<prog>)) = <prog>.

• Better error messages.

• Attributes.

• Type suffixes.

[tqchen]

I am not sure if this is staled already, close this for now because this issue is rather inactive. Let us open new thread with smaller actionable items