fix isnan_v2 is not supported in paddle2onnx (#14060)

Author: GreatV

ppocr/modeling/heads/det_db_head.py

@@ -76,10 +76,12 @@ def __init__(self, in_channels, kernel_list=[3, 2, 2], **kwargs):
     def forward(self, x, return_f=False):
         x = self.conv1(x)
         x = self.conv_bn1(x)
-        x = paddle.where(paddle.isnan(x), paddle.zeros_like(x), x)
+        if self.training:
+            x = paddle.where(paddle.isnan(x), paddle.zeros_like(x), x)
         x = self.conv2(x)
         x = self.conv_bn2(x)
-        x = paddle.where(paddle.isnan(x), paddle.zeros_like(x), x)
+        if self.training:
+            x = paddle.where(paddle.isnan(x), paddle.zeros_like(x), x)
         if return_f is True:
             f = x
         x = self.conv3(x)