python: add `CHECK-LABEL` statements to localize checks (#1363)

It seems as though an upstream change in PyTorch has caused the module
dump to include not just the module being tested, but also several
seemingly unrelated functions in the `torch._decom.decompositions`
namespace.  The presence of these new functions caused lit to match
variables against incorrect statements (i.e.  statements in the
unrelated functions instead of the module under test).

This patch inserts `CHECK-LABEL` statements in the failing tests so that
lit ignores these unrelated functions and only checks the statements at
or after the test module definition.

test/python/importer/jit_ir/ivalue_import/object-identity-torch-bug.py

@@ -18,6 +18,7 @@ mb = ModuleBuilder()
 # `torch.Tensor` is just a pointer to a TensorImpl under the hood, and so
 # naively duplicating a Tensor retains the identity of the TensorImpl.
 
+# CHECK-LABEL: torch.class_type @__torch__.TestModule {
 class TestModule(torch.nn.Module):
     def __init__(self):
         super().__init__()

test/python/importer/jit_ir/ivalue_import/quantization.py

@@ -12,6 +12,7 @@ from torch_mlir.dialects.torch.importer.jit_ir import ModuleBuilder
 
 mb = ModuleBuilder()
 
+# CHECK-LABEL: torch.class_type @__torch__.TestModule {
 class TestModule(torch.nn.Module):
     def __init__(self):
         super().__init__()

test/python/importer/jit_ir/ivalue_import/submodules.py

@@ -22,6 +22,7 @@ class TestModule(torch.nn.Module):
         self.s0 = Submodule(0)
         self.s1 = Submodule(1)
 
+# CHECK-LABEL: torch.class_type @__torch__.TestModule {
 # CHECK:         %[[T:.*]] = torch.constant.bool true
 
 # CHECK:         %[[N0:.*]] = torch.constant.int 0