load instantiate model by DISPLAY_NAME; current usage is by Model/Artifact ID.

Current MODEL load via:
```
model = aiplatform.Model(
model_name=model_id,
project=PROJECT,
version="1",
)
```
requires user to obtain model ID such as:
```
model_id = aiplatform.Model.list(
filter=("display_name={}")
.format(DISPLAY_NAME)
)[0].name
```
which return artifact/model ID by the `DISPLAY_NAME`. It would be really optimal to be able to query the model by its NAME, which seems (by documentation) was the case in previous `google.cloud.aiplatform` releases. The most recent release, however, does not allow querying by NAME, instead require particular MODEL/ARTIFACT ID, which by some might be perceived as awkward. Having query by NAME would be really convenient. Thanks!