[TVMScript] Comments and docstrings printing

[cyx-6]

This PR introduces the CommentDoc for comments printing and DocStringDoc for docstring printing. It enables to add free comments and docstring as stmt in printing, e.g.

# comment 1
# comment 2
"""
docstring 1
docstring 2
"""

The free here means to not be bound to any stmt, but acts as a single stmt, similar to ExprStmtDoc for ExprDoc.

This PR also introduces an example for the CommentDoc, as follow up of #13819.
In the old printer, we always print a # with T.block("root"):, when there is an implicit root block skipped when printing. For example,

@T.prim_func
def main():
  # with T.block("root"):
  a = T.alloc_buffer((128, 128))
  for i, j in T.grid(128, 128):
    with T.block(""):
      ...

We bring this syntax reminder back in this PR.
In addition, we introduce a field of ir_usage and print_headers into the printer configuration, to support the printing of headers for IRModule and PrimFunc. For example,

# from tvm.script import ir as I
# from tvm.script import tir as T

@I.ir_module
class Module():
  @T.prim_func
  def func():
    ...

[tvm-bot]

Thanks for contributing to TVM! Please refer to the contributing guidelines https://tvm.apache.org/docs/contribute/ for useful information and tips. Please request code reviews from Reviewers by @-ing them in a comment.

• cc @junrushao _{See #10317 for details}

_{Generated by tvm-bot}

[junrushao]

Thanks for brining in this change!

BTW, for IRModule and PrimFunc printing, would you like to add a couple of lines of comments for clarify:

# from tvm.script import ir as I      # if `IR(...)` is ever used
# from tvm.script import tir as T   # if `TIR(...)` is ever used

@I.ir_module
class Module()

[junrushao]

Perfect. Please address my comment above and fix the lint, and we are good to go!