add onnx dependency for x2paddle (#1160)

* add denpendency

* add onnx dependency

* add onnx requirement

requirements.txt

@@ -12,3 +12,4 @@ multiprocess
 packaging
 x2paddle
 rarfile
+onnx >= 1.6.0
\ No newline at end of file

visualdl/component/inference/model_convert_server.py

@@ -59,7 +59,7 @@ class ModelConvertApi(object):
                             import onnx  # noqa: F401
                         except Exception:
                             raise RuntimeError(
-                                "[ERROR] onnx is not installed, use \"pip install onnx==1.6.0\"."
+                                "[ERROR] onnx is not installed, use \"pip install onnx>=1.6.0\"."
                             )
                         onnx2paddle(fp.name, tmpdirname)
                     elif format == 'caffe':