[fx] skip diffusers unitest if it is not installed (#1799)

requirements/requirements-test.txt

-diffusers
 fbgemm-gpu==0.2.0
 pytest
 torchvision

tests/test_fx/test_tracer/test_hf_model/test_hf_diffuser.py

-import diffusers
 import pytest
 import torch
-import transformers
 from torch.fx import GraphModule
 from utils import trace_model_and_compare_output
 
+import transformers
 from colossalai.fx import ColoTracer
 
+try:
+    import diffusers
+    HAS_DIFFUSERS = True
+except ImportError:
+    HAS_DIFFUSERS = False
+
 BATCH_SIZE = 2
 SEQ_LENGTH = 5
 HEIGHT = 224
@@ -16,6 +21,7 @@ LATENTS_SHAPE = (BATCH_SIZE, IN_CHANNELS, HEIGHT // 8, WIDTH // 8)
 TIME_STEP = 2
 
 
+@pytest.mark.skipif(not HAS_DIFFUSERS, reason="diffusers has not been installed")
 def test_vae():
     MODEL_LIST = [
         diffusers.AutoencoderKL,
@@ -80,6 +86,7 @@ def test_clip():
         trace_model_and_compare_output(model, data_gen)
 
 
+@pytest.mark.skipif(not HAS_DIFFUSERS, reason="diffusers has not been installed")
 @pytest.mark.skip(reason='cannot pass the test yet')
 def test_unet():
     MODEL_LIST = [