[Fix][TVMScript] Fix LetStmt printing logic

[cyx-6]

This PR is the bug fix reported in #13892. Initially, we mix the logic of LetStmt docsifying method with and without concise scoping. For example, in

x = T.var("int32")
with T.let(x, 0):

x in the LetStmt works as a right value, while in

x: T.int32 = 0

x in the LetStmt works as a left value as result.
Our old logic mixed them together to generate the wrong code for the first case.
Meanwhile, during the fix, we found another bug in concise scoping check. For example, we have

x = T.var("int32")
y = T.var("int32")
with T.let(x, y):
  with T.let(y, 0):

here we should not output

x = T.var("int32")
y = T.var("int32")
with T.let(x, y):
  y: int32 = 0

becase this will define a new y_1: int32 = 0 indeed, due the the variable shadowing logic of the parser, which is different from the y we define and refer to.
Our concise scoping v: ... = ... should launch if and only if the v is never defined before.
Otherwise, we use with T.let(v, ...): instead.

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[junrushao]

CC: @wrongtest-intellif @lightzhan-intellif

[wrongtest-intellif]

x = T.var("int32")
y = T.var("int32")
with T.let(x, y):
  with T.let(y, 0):

Thank you for the fix! Here is my small question for the example, does it shoud be

with T.let(y, 0):
  with T.let(x, y):

[junrushao]

Off the topic, in the future, we might want to change the syntax to:

with T.LetStmt(a) as b:

[cyx-6]

@wrongtest-intellif Oh, that is just an example of the case that a LetStmt->var - y has usage before the its new value. I picked the outer LetStmt->value field for the usage of y for convenience. So never mind its logic, it makes no sense, except its format.

[lightzhan-intellif]

Off the topic, in the future, we might want to change the syntax to:

with T.LetStmt(a) as b:

I prefer this. The current one looks a little bit confusing from the view of native python users.

[junrushao]

Sounds good! I am merging this bugfix in first given it seems non-controversial.

Right now I am focusing on implementing TVMScript printer for Relax (coming soon). And after this is done, the syntax we mentioned above will be introduced with backward compatibility guarantee.