[fx2trt] flatten converter (#62202)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/62202
Add acc_ops.flatten converter. Also migrate to oss acc tacer for trt interpreter.
Test Plan: unit test
Reviewed By: khabinov
Differential Revision: D29861555
fbshipit-source-id: dac88a703fdbf386f3f7fb27674a67951f3add49
