[tests] remove TF tests (uses of require_tf) (#38944)

* remove uses of require_tf

* remove redundant import guards

* this class has no tests

* nits

* del tf rng comment

tests/pipelines/test_pipelines_depth_estimation.py

@@ -24,7 +24,6 @@ from transformers.testing_utils import (
     compare_pipeline_output_to_hub_spec,
     is_pipeline_test,
     nested_simplify,
-    require_tf,
     require_timm,
     require_torch,
     require_vision,
@@ -123,11 +122,6 @@ class DepthEstimationPipelineTests(unittest.TestCase):
         for single_output in outputs:
             compare_pipeline_output_to_hub_spec(single_output, DepthEstimationOutput)
 
-    @require_tf
-    @unittest.skip(reason="Depth estimation is not implemented in TF")
-    def test_small_model_tf(self):
-        pass
-
     @slow
     @require_torch
     def test_large_model_pt(self):