Fix a bug in retrieve model (ml4ai#331)

If the model path is None or doesn't exist when calling the retrieve_model
function, download the default model from the server.

skema/img2mml/api.py

@@ -25,8 +25,8 @@ def retrieve_model(model_path=None) -> str:
     cwd = Path(__file__).parents[0]
     MODEL_BASE_ADDRESS = "https://artifacts.askem.lum.ai/skema/img2mml/models"
     MODEL_NAME = "cnn_xfmer_arxiv_im2mml_with_fonts_boldface_best.pt"
-
-    if model_path is None:
+    # If the model path is none or doesn't exist, the default model will be downloaded from server.
+    if model_path is None or not os.path.exists(model_path):
         model_path = cwd / "trained_models" / MODEL_NAME
 
         # Check if the model file already exists