code: def fill_na(df, val):
      train_df = df.fillna(value=val)
      return train_df
  
imports: import bigdl.orca.data.pandas
  from bigdl.orca.data.transformer import *
label_desc: Use image node_id 4 for calling a global function with example usage: fill_na(df, val) and returns: train_df