[rllib] Add test case that we don't have a hard torch dependency (#7926)

rllib/utils/framework.py

@@ -101,7 +101,10 @@ def try_import_tfp(error=False):
 
 # Fake module for torch.nn.
 class NNStub:
-    pass
+    def __init__(self, *a, **kw):
+        # Fake nn.functional module within torch.nn.
+        self.functional = None
+        self.Module = ModuleStub
 
 
 # Fake class for torch.nn.Module to allow it to be inherited from.
@@ -120,7 +123,7 @@ def try_import_torch(error=False):
     """
     if "RLLIB_TEST_NO_TORCH_IMPORT" in os.environ:
         logger.warning("Not importing Torch for test purposes.")
-        return None, None
+        return _torch_stubs()
 
     try:
         import torch
@@ -129,10 +132,12 @@ def try_import_torch(error=False):
     except ImportError as e:
         if error:
             raise e
+        return _torch_stubs()
 
-        nn = NNStub()
-        nn.Module = ModuleStub
-        return None, nn
+
+def _torch_stubs():
+    nn = NNStub()
+    return None, nn
 
 
 def get_variable(value,
@@ -165,7 +170,7 @@ def get_variable(value,
         return tf.compat.v1.get_variable(
             tf_name, initializer=value, dtype=dtype, trainable=trainable)
     elif framework == "torch" and torch_tensor is True:
-        import torch
+        torch, _ = try_import_torch()
         var_ = torch.from_numpy(value)
         var_.requires_grad = trainable
         return var_