fix(specdec): generate all multimodal sources and pad VLM batches

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✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@                            Coverage Diff                            @@
##             haoguo/dflash-cosmos3-launcher-example    #2195   +/-   ##
=========================================================================
  Coverage                                          ?   77.76%           
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  Files                                             ?      521           
  Lines                                             ?    61264           
  Branches                                          ?        0           
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  Hits                                              ?    47640           
  Misses                                            ?    13624           
  Partials                                          ?        0           

Flag	Coverage Δ
examples	43.02% <ø> (?)
gpu	58.62% <ø> (?)
regression	14.97% <ø> (?)

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