[RELAY] IR builder stablize refactor, clean pass

[tqchen]

This is a clean refactor PR to attempt stablize the relay python IR building API, it also contains fixes and patches to enable text format.

One of the general guidelines behind this refactor are:

• Minimum abstraction when possible (simplification of IRBuilder)
• Python API consistency (env.merge->env.update)
• Make ir_pass style consistent
  • Expr is always the first argument
  • Not require env to be present, when necessary, pass as a second argument

List of Changes

• Simplify ir_builder to scope_builder cc @jroesch
  • ScopeBuilder is only used to build if and let scopes.
  • Use relay.Function to build functions (single way to build functions)
• Rewrite all the test cases with the new style.
• Add text print test to each op, fix problems that surface due to the new testcases.
• Improve the BroadcastRel to handle symbolic case cc @jroesch
• Fix TupleWrapper cc @slyubomirsky
  • To make sure python unpack works, we need to throw index error when there is out of bound access
  • TupleWrapper cannot assume the input field is a tuple type
• Enable TulpleGetItem type inference with incomplete type
• Fix dead code elimination when type is bool cc @MarisaKirisame
• Fix LeakyReluAttrs cc @siju-samuel
• Merge reduce tests into a more concise functioncc @siju-samuel
• Rename TypeParam->TypeVar as suggested by @joshpoll

[tqchen]

@jroesch @MarisaKirisame @junrushao1994 @slyubomirsky please review

[junrushao]

Will do tonight

[MarisaKirisame]

const std::string& name

[MarisaKirisame]

do XXXX