[TVMScript] Preserve LetStmt of constants (#14531)

* [TVMScript] Preserve LetStmt of constants

Prior to this commit, the `bind_assign_value` implementation for TIR
would treat assignment of constants (e.g. `j = 42`) as meta-variables
to be inserted at their point of use.  This commit updates the parsing
to treat `j = 42` as a shorthand for the TIR `LetStmt`, similar to the
update made in #14320.  This
behavior is more consistent with the other uses of `bind_assign_value`
as assignment, with `j = T.meta_var(42)` used to represent
meta-variables.

* Updated tests that relied on implicit meta_var

python/tvm/script/parser/tir/parser.py

@@ -143,14 +143,14 @@ def bind_assign_value(self: Parser, node: doc.expr, var_name: str, value: Any) -
     ):
         IRBuilder.name(var_name, value)
         return value
-    elif isinstance(value, PrimExpr):
+    else:
+        value = tvm.runtime.convert(value)
         frame = T.LetStmt(value)
         var = frame.var
         IRBuilder.name(var_name, var)
         frame.add_callback(partial(frame.__exit__, None, None, None))
         frame.__enter__()
         return var
-    return value
 
 
 @dispatch.register(token="tir", type_name="For")

tests/python/unittest/test_arith_domain_touched.py

@@ -22,7 +22,7 @@
 @T.prim_func
 def scalar_func(a: T.handle, b: T.handle):
     m = T.int32()
-    n = 100
+    n = T.meta_var(100)
     A = T.match_buffer(a, (n, m))
     B = T.match_buffer(b, (n, m))
 

tests/python/unittest/test_tvmscript_syntax_sugar.py

@@ -253,8 +253,8 @@ def func_without_type_annotation(A: T.Buffer((1,), "int32")):
 def test_letstmt_bind_with_constant():
     @T.prim_func
     def constant_binds():
-        x = 1
-        y = 42.0
+        x = T.meta_var(1)
+        y = T.meta_var(42.0)
         T.evaluate(T.cast(x, "float32") + y)
 
     @T.prim_func
@@ -414,6 +414,21 @@ def implicit(i: T.int32):
     assert_structural_equal(implicit, explicit)
 
 
+def test_preserve_trivial_let_binding_of_value():
+    @T.prim_func
+    def explicit(i: T.int32):
+        j = T.int32()
+        T.LetStmt(42, var=j)
+        T.evaluate(j)
+
+    @T.prim_func
+    def implicit(i: T.int32):
+        j = 42
+        T.evaluate(j)
+
+    assert_structural_equal(implicit, explicit)
+
+
 def test_preserve_parameter_name():
     @T.prim_func
     def func(i: T.int32):