[AutoLayoutModel] Outdated docs and silent error

[falcaopetri]

Describe the bug
README contains the following (since #73):

Perform DL layout detection in 4 lines of code

import layoutparser as lp
model = lp.AutoLayoutModel('lp://EfficientDete/PubLayNet')
# image = Image.open("path/to/image")
layout = model.detect(image) 

This fails to instantiate a valid model because AutoLayoutModel runs

layout-parser/src/layoutparser/models/auto_layoutmodel.py

Lines 62 to 63 in 89aca03

	for backend_name in ALL_AVAILABLE_BACKENDS:
	if backend_name in config_path:

and

>>> import layoutparser as lp
>>> list(lp.models.auto_layoutmodel.ALL_AVAILABLE_BACKENDS)
['detectron2', 'paddledetection', 'efficientdet']

i.e., 'efficientdet' is not in 'lp://EfficientDete/PubLayNet'.

When AutoLayoutModel cannot find a model to instantiate, it silently returns None (because it returns nothing).
This means that we get AttributeError: 'NoneType' object has no attribute 'detect' during layout = model.detect(image)

My suggestions are:

• update the README with a proper model, perhaps from:
  

  layout-parser/tests/test_model.py

  Line 50 in f230971

  "lp://efficientdet/PubLayNet/tf_efficientdet_d0/config",

• AutoLayoutModel should complain if it fails auto detecting the model.
• add # from PIL import Image along README's # image = Image.open("path/to/image")

[lolipopshock]

Thanks -- Good catch! Will fix it later

[lolipopshock]

Also see #86