Avoid OOM in doctest CI (#24139)

* fix

* fix

---------
Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>

src/transformers/testing_utils.py

@@ -1870,9 +1870,18 @@ def preprocess_string(string, skip_cuda_tests):
         ):
             is_cuda_found = True
             break
+
     modified_string = ""
     if not is_cuda_found:
         modified_string = "".join(codeblocks)
+
+        if ">>>" in modified_string:
+            lines = modified_string.split("\n")
+            indent = len(lines[-1]) - len(lines[-1].lstrip())
+
+            cleanup = ">>> import gc; gc.collect()  # doctest: +IGNORE_RESULT"
+            modified_string += "\n" + " " * indent + cleanup
+
     return modified_string