[Vertex AI] Add ImagenModel with generateImages functions
#794
| Job | Run time |
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| 2m 41s | |
| 1m 0s | |
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| 3m 30s | |
| 1m 56s | |
| 5m 55s | |
| 2m 1s | |
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| 2m 7s | |
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| 3m 7s | |
| 1m 47s | |
| 2m 51s | |
| 36m 33s |