jekins failed to catch failure of LSTMSerialTest

[dding3]

It's werid that lstm failed to save(https://github.com/intel-analytics/BigDL/issues/5200) while we have ut to cover the serialize test(https://github.com/intel-analytics/BigDL/blob/main/scala/dllib/src/test/scala/com/intel/analytics/bigdl/dllib/keras/layers/LSTMSpec.scala#L94) and our jekins can pass.

I tried to run the LSTMSerialTest on local and it also failed.

[Le-Zheng]

http://10.112.231.51:18888/job/BigDL-PRVN-scalatest-zoo-Spark-2.4/577/testReport/com.intel.analytics.bigdl.dllib.keras.layers/LSTMSpec/ Jenkins test passes exactly.

[Le-Zheng]

And I obtained the same result as jenkins when running this UT on my machine.