model_ckpt_conversion: use python module, enable gcloud checkpoint downloading

Author: karan

jetstream/tools/maxtext/model_ckpt_conversion.sh

@@ -69,7 +69,7 @@ else
 
     path_parts=(${CHKPT_BUCKET//\// })
     directory_substring=${path_parts[-1]}
-    CONVERT_CKPT_SCRIPT="llama_or_mistral_ckpt.py"
+    CONVERT_CKPT_SCRIPT="llama_or_mistral_ckpt"
 
     if [[ ! -z "${LORA_INPUT_ADAPTERS_PATH}" ]]; then
 	lora_local_path="/tmp/"