Fix issue in TrainMnistWithLSTM

[KexinFeng]

Description

This fixes issue #1936. An unsupported exception is thrown, since the MXNet engine LSTM model does not have the feature of excluding bias. See https://mxnet.apache.org/versions/master/api/python/docs/api/npx/generated/mxnet.npx.rnn.html

[codecov-commenter]

Codecov Report

Merging #1965 (c2b829b) into master (bb5073f) will decrease coverage by 2.05%.
The diff coverage is 67.39%.

@@             Coverage Diff              @@
##             master    #1965      +/-   ##
============================================
- Coverage     72.08%   70.03%   -2.06%     
- Complexity     5126     5819     +693     
============================================
  Files           473      574     +101     
  Lines         21970    25827    +3857     
  Branches       2351     2781     +430     
============================================
+ Hits          15838    18088    +2250     
- Misses         4925     6373    +1448     
- Partials       1207     1366     +159     

Impacted Files	Coverage Δ
api/src/main/java/ai/djl/modality/cv/Image.java	69.23% <ø> (-4.11%)	⬇️
...rc/main/java/ai/djl/modality/cv/MultiBoxPrior.java	76.00% <ø> (ø)
...rc/main/java/ai/djl/modality/cv/output/Joints.java	71.42% <ø> (ø)
.../main/java/ai/djl/modality/cv/output/Landmark.java	100.00% <ø> (ø)
...main/java/ai/djl/modality/cv/output/Rectangle.java	72.41% <0.00%> (ø)
...i/djl/modality/cv/translator/BigGANTranslator.java	21.42% <0.00%> (-5.24%)	⬇️
...odality/cv/translator/BigGANTranslatorFactory.java	33.33% <0.00%> (+8.33%)	⬆️
...nslator/InstanceSegmentationTranslatorFactory.java	14.28% <0.00%> (-3.90%)	⬇️
.../cv/translator/SemanticSegmentationTranslator.java	0.00% <0.00%> (ø)
.../cv/translator/StyleTransferTranslatorFactory.java	40.00% <ø> (ø)
... and 477 more

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