[contrib] some cases of transducer's test_transducer_joint.py broken

[crcrpar]

Describe the Bug

Minimal Steps/Code to Reproduce the Bug

$ pytest test_transducer_joint.py

...

FAILED transducer/test_transducer_joint.py::TransducerJointTest::test_transducer_joint_pack_relu_dropout - RuntimeError: one of the variables needed for gradient compu...
FAILED transducer/test_transducer_joint.py::TransducerJointTest::test_transducer_joint_relu_dropout - RuntimeError: one of the variables needed for gradient computatio...
FAILED transducer/test_transducer_joint.py::TransducerJointTest::test_transducer_joint_vec_pack_relu_dropout - RuntimeError: one of the variables needed for gradient c...
FAILED transducer/test_transducer_joint.py::TransducerJointTest::test_transducer_joint_vec_relu_dropout - RuntimeError: one of the variables needed for gradient comput...

Expected Behavior

Environment

• A100
• Ubuntu 20.04
• Python 3.8
• PyTorch: 1.13.0a0