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!

Start with the Model constructor and Model.list usage shown in the issue, then review the current model-loading API and its documentation for how display_name and model_name are handled. Done means the SDK supports the requested display-name lookup or clearly documents the supported alternative, with coverage for the demonstrated use case.

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