A precheck function in customize.py

[arjunsuresh]

We do have cases where we want to turn on some dependencies based on some input validation. For example, if a model path is given and we might want to validate the presence of model (say using md5sum check) and only if it fails turn on the model download dependency. Currently this is possible via doing the check in preprocess function and doing the download as a prehook dependency. It'll be a cleaner approach if we can do this check in a precheck function and use normal dependencies instead of prehook dependencies.