title: fastai: Evaluate tabular prediction model with pre-splitted dataset
body: <p>Given a pre-splitted dataset for training and testing, I am wondering how to apply the prediction in fastai accordingly to access MAE and RMSE values.</p>
  <p>The following example is from fastai and slightly modified with the train_test_split from sklearn.</p>
  <pre><code>import numpy as np
  from sklearn.model_selection import train_test_split
  from fastai.tabular.all import *
  import pandas as pd
  
  path = untar_data(URLs.ADULT_SAMPLE)
  df = pd.read_csv(path/'adult.csv')
  
  train, test = train_test_split(df, test_size=0.20, random_state=42)
  
  cat_names = ['workclass', 'education', 'marital-status', 'occupation', 'relationship', 'race']
  cont_names = ['age', 'fnlwgt', 'education-num']
  procs = [Categorify, FillMissing, Normalize]
  dls = TabularDataLoaders.from_df(train, path, procs=procs, cat_names=cat_names, cont_names=cont_names, 
                                   y_names=&quot;salary&quot;)
  learn = tabular_learner(dls)
  
  
  learn.fit_one_cycle(5)
  
  epoch   train_loss  valid_loss  time
  0   0.378432    0.356029    00:05
  1   0.369692    0.358837    00:05
  2   0.355757    0.348524    00:05
  3   0.342714    0.348011    00:05
  4   0.334072    0.346690    00:05
  
  
  learn.unfreeze()
  learn.fit_one_cycle(10, max_lr=slice(10e-4, 10e-3))
  
  epoch   train_loss  valid_loss  time
  0   0.343953    0.350457    00:05
  1   0.349379    0.353308    00:04
  2   0.360508    0.352564    00:04
  3   0.338458    0.351742    00:05
  4   0.334585    0.352128    00:05
  5   0.342312    0.351003    00:04
  6   0.329152    0.350455    00:05
  7   0.334460    0.351833    00:05
  8   0.328608    0.351415    00:05
  9   0.333205    0.352079    00:04
  </code></pre>
  <p>Now how can I apply the learn model to my test set to compute my metrics? Something like the following is not working for me:</p>
  <pre><code>learn.predict(test)
  </code></pre>
  <p>Here I get the following Error: <code>AttributeError: 'DataFrame' object has no attribute 'to_frame'</code></p>
  <p>Thanks for your help in advance!</p>