Replace EagerTensor with Tensor

[JiabinYang]

PR types

Function optimization

PR changes

Others

Describe

This PR using Tensor to replace EagerTensor to optimize eager performance in most cases.
Some important changes:

1. Make EagerTensor a simple wrapper of variable (with name and framework::Variable )
2. Support pten::SelectedRows in Tensor and support grad accumulation of pten::SelectedRows and pten::DenseTensor
3. Make new EagerTensor a friend class of pten::SelectedRows to reconstruct it from framework::Variable to Tensor
4. Adjust all supported API with Tensor
5. Update code gene for mid and final state
6. Optimize some utils method in EagerUtils to adjust new data structure.

[paddle-bot-old]

Thanks for your contribution!
Please wait for the result of CI firstly. See Paddle CI Manual for details.

[chenwhql]

LGTM for PADDLE_ENFORCE

[XieYunshen]

LGTM for set_tests_properties(test_imperative_lod_tensor_to_selected_rows PROPERTIES TIMEOUT 200) set_tests_properties(test_imperative_optimizer_v2 PROPERTIES TIMEOUT 150)