check for tpu availability in save_pretrained (#7699)

Added is_torch_tpu_available() to the condition
for saving a model as xla model. "xla_device"
property of config can also be True on a non-xla
device, when loading a checkpointthat was trained
on xla before.

Resolves #7695

src/transformers/modeling_utils.py

@@ -716,7 +716,7 @@ class PreTrainedModel(nn.Module, ModuleUtilsMixin, GenerationMixin):
         # If we save using the predefined names, we can load using `from_pretrained`
         output_model_file = os.path.join(save_directory, WEIGHTS_NAME)
 
-        if getattr(self.config, "xla_device", False):
+        if getattr(self.config, "xla_device", False) and is_torch_tpu_available():
             import torch_xla.core.xla_model as xm
 
             if xm.is_master_ordinal():