[Frontend] [Tensorflow2] Added test infrastructure for TF2 frozen models (#8074)

* added test infrastructure for frozen TF2 models

* linting with black

* removing some comments

* change in comment in sequential test

* addressed the comments

* refactored to place vmobj_to_list in a common file

* Added helper function in python/tvm/relay/testing/tf.py

Co-authored-by: David Huang <davhuan@amazon.com>
Co-authored-by: Rohan Mukherjee <mukrohan@amazon.com>
Co-authored-by: Xiao <weix@amazon.com>

* Refactor tf according to CI error

Co-authored-by: David Huang <davhuan@amazon.com>
Co-authored-by: Rohan Mukherjee <mukrohan@amazon.com>
Co-authored-by: Xiao <weix@amazon.com>

* Added docstring

Co-authored-by: David Huang <davhuan@amazon.com>
Co-authored-by: Rohan Mukherjee <mukrohan@amazon.com>
Co-authored-by: Xiao <weix@amazon.com>

* removing print

Co-authored-by: David Huang <davhuan@amazon.com>
Co-authored-by: Xiao <weix@amazon.com>

python/tvm/relay/testing/tf.py

@@ -28,6 +28,8 @@ import numpy as np
 # Tensorflow imports
 import tensorflow as tf
 from tensorflow.core.framework import graph_pb2
+
+import tvm
 from tvm.contrib.download import download_testdata
 
 try:
@@ -73,6 +75,46 @@ def convert_to_list(x):
     return x
 
 
+def vmobj_to_list(o):
+    """Converts TVM objects returned by VM execution to Python List.
+
+    Parameters
+    ----------
+    o : Obj
+        VM Object as output from VM runtime executor.
+
+    Returns
+    -------
+    result : list
+        Numpy objects as list with equivalent values to the input object.
+
+    """
+
+    if isinstance(o, tvm.nd.NDArray):
+        result = [o.asnumpy()]
+    elif isinstance(o, tvm.runtime.container.ADT):
+        result = []
+        for f in o:
+            result.extend(vmobj_to_list(f))
+    elif isinstance(o, tvm.relay.backend.interpreter.ConstructorValue):
+        if o.constructor.name_hint == "Cons":
+            tl = vmobj_to_list(o.fields[1])
+            hd = vmobj_to_list(o.fields[0])
+            hd.extend(tl)
+            result = hd
+        elif o.constructor.name_hint == "Nil":
+            result = []
+        elif "tensor_nil" in o.constructor.name_hint:
+            result = [0]
+        elif "tensor" in o.constructor.name_hint:
+            result = [o.fields[0].asnumpy()]
+        else:
+            raise RuntimeError("Unknown object type: %s" % o.constructor.name_hint)
+    else:
+        raise RuntimeError("Unknown object type: %s" % type(o))
+    return result
+
+
 def AddShapesToGraphDef(session, out_node):
     """Add shapes attribute to nodes of the graph.
         Input graph here is the default graph in context.