Update rec_nrtr_head.py (#8108)

fix bug when export nrtr model with paddlepaddle develop version

ppocr/modeling/heads/rec_nrtr_head.py

@@ -162,7 +162,7 @@ class Transformer(nn.Layer):
             memory = src
         dec_seq = paddle.full((bs, 1), 2, dtype=paddle.int64)
         dec_prob = paddle.full((bs, 1), 1., dtype=paddle.float32)
-        for len_dec_seq in range(1, self.max_len):
+        for len_dec_seq in range(1, paddle.to_tensor(self.max_len)):
             dec_seq_embed = self.embedding(dec_seq)
             dec_seq_embed = self.positional_encoding(dec_seq_embed)
             tgt_mask = self.generate_square_subsequent_mask(
@@ -304,7 +304,7 @@ class Transformer(nn.Layer):
             inst_idx_to_position_map = get_inst_idx_to_tensor_position_map(
                 active_inst_idx_list)
             # Decode
-            for len_dec_seq in range(1, self.max_len):
+            for len_dec_seq in range(1, paddle.to_tensor(self.max_len)):
                 src_enc_copy = src_enc.clone()
                 active_inst_idx_list = beam_decode_step(
                     inst_dec_beams, len_dec_seq, src_enc_copy,