Inconsistent GOOGLE_CLOUD_PROJECT env variable

Currently in the [google/cloud/aiplatform/initializer.py#L92](https://github.com/googleapis/python-aiplatform/blob/c81eb7219631e58c6002a7aff8905545bda1132a/google/cloud/aiplatform/initializer.py#L92) we expect GOOGLE_CLOUD_PROJECT variable to be set to project number (and we try to convert it to project Id further).

But [google.auth.default()](https://github.com/googleapis/google-auth-library-python/blob/main/google/auth/_default.py) we rely on everywhere actually expects GOOGLE_CLOUD_PROJECT to be set to project Id already.

So having that env variable set gives wrong values for [project property](https://github.com/googleapis/python-aiplatform/blob/c81eb7219631e58c6002a7aff8905545bda1132a/google/cloud/aiplatform/initializer.py#L350) from initialized context.

This can probably have side effects on the code dependent on this library, but we can either introduce another env variable for project number, or remove the [code trying to convert it to project Id](https://github.com/googleapis/python-aiplatform/blob/c81eb7219631e58c6002a7aff8905545bda1132a/google/cloud/aiplatform/initializer.py#L101).

从 google/cloud/aiplatform/initializer.py 中大约第 92、101 和 350 行开始，然后将其对 GOOGLE_CLOUD_PROJECT 的处理与 google.auth.default() 进行比较。确定初始化的上下文应公开哪个值，并选择一致的环境变量处理方式。当设置 GOOGLE_CLOUD_PROJECT 时，项目属性不再产生错误值，即表示完成。

由索引模型根据 Issue 内容生成。