Fix MyPY (#19560)

### Summary
From the pytorch code, create_symintnode will always return a symint
when the sym input is symbolic. Mypy doesn't know this. Assert is an
idiomatic way to express the narrowing to mypy. This is also in test
code, so low risk.

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218 @mansnils
@Sebastian-Larsson @robell @rascani

Author: GregoryComer

backends/arm/test/misc/test_tosa_dialect_resize.py

@@ -22,6 +22,7 @@ def _make_symint(
     shape_env: ShapeEnv, symbol: str, hint: int, min: int = 1, max: int = 64
 ) -> torch.SymInt:
     symint = shape_env.create_symintnode(sympy.Symbol(symbol), hint=hint)
+    assert isinstance(symint, torch.SymInt)
     shape_env.constrain_symbol_range(
         symint.node.expr, compiler_min=min, compiler_max=max
     )

backends/arm/test/misc/test_tosa_dialect_shape_ops.py

@@ -23,6 +23,7 @@ def _make_symint(
 ) -> torch.SymInt:
     """Create a symbolic dimension backed by the provided ShapeEnv."""
     symint = shape_env.create_symintnode(sympy.Symbol(symbol), hint=hint)
+    assert isinstance(symint, torch.SymInt)
     symbol_expr = symint.node.expr
     shape_env.constrain_symbol_range(symbol_expr, compiler_min=min, compiler_max=max)
     return symint

backends/arm/test/passes/test_rewrite_upsample_pass.py

@@ -14,6 +14,7 @@ def _make_symint(
     shape_env: ShapeEnv, symbol: str, hint: int, min: int = 1, max: int = 64
 ) -> torch.SymInt:
     symint = shape_env.create_symintnode(sympy.Symbol(symbol), hint=hint)
+    assert isinstance(symint, torch.SymInt)
     shape_env.constrain_symbol_range(
         symint.node.expr, compiler_min=min, compiler_max=max
     )

backends/arm/test/passes/test_symbolic_value_range.py

@@ -20,6 +20,7 @@ def _make_shape_env(
 ) -> tuple[ShapeEnv, torch.SymInt]:
     shape_env = ShapeEnv()
     symint = shape_env.create_symintnode(sympy.Symbol(symbol_name), hint=hint)
+    assert isinstance(symint, torch.SymInt)
     shape_env.constrain_symbol_range(
         symint.node.expr,
         compiler_min=compiler_min,