fix import bug with relative paths

[IShengFang]

The original version is absolute import.
It causes error during import from outside.
I change it with explicit relative import.

I also fixed bug of "PerceptualLoss no attribute '_forward_pre_hooks' " in init.py

[richzhang]

Thanks for the PerceptualLoss fix.

I end up getting import errors when trying this.

  File "compute_dists_dirs.py", line 5, in <module>
    import models.dist_model as dm
  File "/mnt/src/PerceptualSimilarity/models/dist_model.py", line 14, in <module>
    from .base_model import BaseModel
  File "/mnt/src/PerceptualSimilarity/models/base_model.py", line 3, in <module>
    from ..util import util
ValueError: Attempted relative import beyond toplevel package

[IShengFang]

oh
I am sorry that I didn't test the test code under the repository.
I repacked module and fixed bug[1].
[1] #25 (comment)

[richzhang]

Thanks for your PR! Somehow this got fixed at some point (I don't remember when). There is also a pip install lpips now, feel free to try it :)