Flax testing should not run the full torch test suite (#10725)

* make flax tests pytorch independent

* fix typo

* finish

* improve circle ci

* fix return tensors

* correct flax test

* re-add sentencepiece

* last tokenizer fixes

* finish maybe now

src/transformers/testing_utils.py

@@ -80,6 +80,7 @@ def parse_int_from_env(key, default=None):
 
 _run_slow_tests = parse_flag_from_env("RUN_SLOW", default=False)
 _run_pt_tf_cross_tests = parse_flag_from_env("RUN_PT_TF_CROSS_TESTS", default=False)
+_run_pt_flax_cross_tests = parse_flag_from_env("RUN_PT_FLAX_CROSS_TESTS", default=False)
 _run_custom_tokenizers = parse_flag_from_env("RUN_CUSTOM_TOKENIZERS", default=False)
 _run_pipeline_tests = parse_flag_from_env("RUN_PIPELINE_TESTS", default=False)
 _run_git_lfs_tests = parse_flag_from_env("RUN_GIT_LFS_TESTS", default=False)
@@ -105,6 +106,25 @@ def is_pt_tf_cross_test(test_case):
             return pytest.mark.is_pt_tf_cross_test()(test_case)
 
 
+def is_pt_flax_cross_test(test_case):
+    """
+    Decorator marking a test as a test that control interactions between PyTorch and Flax
+
+    PT+FLAX tests are skipped by default and we can run only them by setting RUN_PT_FLAX_CROSS_TESTS environment
+    variable to a truthy value and selecting the is_pt_flax_cross_test pytest mark.
+
+    """
+    if not _run_pt_flax_cross_tests or not is_torch_available() or not is_flax_available():
+        return unittest.skip("test is PT+FLAX test")(test_case)
+    else:
+        try:
+            import pytest  # We don't need a hard dependency on pytest in the main library
+        except ImportError:
+            return test_case
+        else:
+            return pytest.mark.is_pt_flax_cross_test()(test_case)
+
+
 def is_pipeline_test(test_case):
     """
     Decorator marking a test as a pipeline test.