add ability to import and edit alternative models online

- !import_model <path/to/model/weights> will import a new model,
  prompt the user for its name and description, write it to the
  models.yaml file, and load it.

- !edit_model <model_name> will bring up a previously-defined model
  and prompt the user to edit its descriptive fields.

Example of !import_model

<pre>
invoke> <b>!import_model models/ldm/stable-diffusion-v1/model-epoch08-float16.ckpt</b>
>> Model import in process. Please enter the values needed to configure this model:

Name for this model: <b>waifu-diffusion</b>
Description of this model: <b>Waifu Diffusion v1.3</b>
Configuration file for this model: <b>configs/stable-diffusion/v1-inference.yaml</b>
Default image width: <b>512</b>
Default image height: <b>512</b>
>> New configuration:
waifu-diffusion:
  config: configs/stable-diffusion/v1-inference.yaml
  description: Waifu Diffusion v1.3
  height: 512
  weights: models/ldm/stable-diffusion-v1/model-epoch08-float16.ckpt
  width: 512
OK to import [n]? <b>y</b>
>> Caching model stable-diffusion-1.4 in system RAM
>> Loading waifu-diffusion from models/ldm/stable-diffusion-v1/model-epoch08-float16.ckpt
   | LatentDiffusion: Running in eps-prediction mode
   | DiffusionWrapper has 859.52 M params.
   | Making attention of type 'vanilla' with 512 in_channels
   | Working with z of shape (1, 4, 32, 32) = 4096 dimensions.
   | Making attention of type 'vanilla' with 512 in_channels
   | Using faster float16 precision
</pre>

Example of !edit_model

<pre>
invoke> <b>!edit_model waifu-diffusion</b>
>> Editing model waifu-diffusion from configuration file ./configs/models.yaml
description: <b>Waifu diffusion v1.4beta</b>
weights: models/ldm/stable-diffusion-v1/<b>model-epoch10-float16.ckpt</b>
config: configs/stable-diffusion/v1-inference.yaml
width: 512
height: 512

>> New configuration:
waifu-diffusion:
  config: configs/stable-diffusion/v1-inference.yaml
  description: Waifu diffusion v1.4beta
  weights: models/ldm/stable-diffusion-v1/model-epoch10-float16.ckpt
  height: 512
  width: 512

OK to import [n]? y
>> Caching model stable-diffusion-1.4 in system RAM
>> Loading waifu-diffusion from models/ldm/stable-diffusion-v1/model-epoch10-float16.ckpt
...
</pre>

ldm/models/diffusion/ddpm.py

@@ -106,7 +106,7 @@ class DDPM(pl.LightningModule):
         ], 'currently only supporting "eps" and "x0"'
         self.parameterization = parameterization
         print(
-            f'   >> {self.__class__.__name__}: Running in {self.parameterization}-prediction mode'
+            f'   | {self.__class__.__name__}: Running in {self.parameterization}-prediction mode'
         )
         self.cond_stage_model = None
         self.clip_denoised = clip_denoised