Update __init__.py

`anthropic` is an optional library to install but `anthropic_llms.py` is always imported.

Add a way to check if the module is imported or not.

lm_eval/models/__init__.py

 from . import huggingface
 from . import openai_completions
-from . import anthropic_llms
 from . import textsynth
 from . import dummy
 
+try:
+    import anthropic
+    from . import anthropic_llms
+except:
+    pass
+
 # TODO: implement __all__