Change theano imports to aesara

[fanshi118]

Thank you for opening a PR!

Here are a few important guidelines and requirements to check before your PR can be merged:

• [ ] There is an informative high-level description of the changes.
• [ ] The description and/or commit message(s) references the relevant GitHub issue(s).
• [ ] The commit messages follow these guidelines.
• [ ] The commits correspond to relevant logical changes.
• [ ] There are tests covering the changes introduced in the PR.

Don't worry, your PR doesn't need to be in perfect order to submit it. As development progresses and/or reviewers request changes, you can always rewrite the history of your feature/PR branches.

If your PR is an ongoing effort and you would like to involve us in the process, simply make it a draft PR.

[fanshi118]

I changed the imports from theano to aesara but left aliases like ts and tt as they are for the moment (will get back to editing those once the tests are working fine). Here is the testing result that I got so far >>

======================================================= short test summary info =======================================================
FAILED tests/test_distributions.py::test_DiscreteMarkovChain_str - TypeError: ufunc 'isfinite' not supported for the input types, an...
FAILED tests/test_distributions.py::test_DiscreteMarkovChain_random - ValueError: Unexpected type in draw_value: <class 'aesara.tens...
FAILED tests/test_distributions.py::test_DiscreteMarkovChain_point - NotImplementedError: Cannot convert gamma_0 ~ Dirichlet to a te...
FAILED tests/test_distributions.py::test_DiscreteMarkovChain_logp - aesara.link.c.exceptions.CompileError: Compilation failed (retur...
FAILED tests/test_distributions.py::test_SwitchingProcess_random - NotImplementedError: Cannot convert TensorConstant{0} to a tensor...
FAILED tests/test_distributions.py::test_PoissonZeroProcess_point - NotImplementedError: Cannot convert Elemwise{Cast{float64}}.0 to...
FAILED tests/test_distributions.py::test_random_PoissonZeroProcess_DiscreteMarkovChain - NotImplementedError: Cannot convert p_0 ~ D...
FAILED tests/test_distributions.py::test_SwitchingProcess - NotImplementedError: Cannot convert TensorConstant{0} to a tensor variable.
FAILED tests/test_distributions.py::test_subset_args - NotImplementedError: Cannot convert TensorConstant{[0.1 1.2 2.3]} to a tensor...
FAILED tests/test_estimation.py::test_only_positive_state - NotImplementedError: Cannot convert p_0 ~ Dirichlet to a tensor variable.
FAILED tests/test_estimation.py::test_time_varying_model - aesara.link.c.exceptions.CompileError: Compilation failed (return status=1):
FAILED tests/test_step_methods.py::test_FFBSStep - TypeError: ufunc 'isfinite' not supported for the input types, and the inputs cou...
FAILED tests/test_step_methods.py::test_FFBSStep_extreme - NotImplementedError: Cannot convert p_0 ~ Dirichlet to a tensor variable.
FAILED tests/test_step_methods.py::test_TransMatConjugateStep - NotImplementedError: Cannot convert p_0 ~ Dirichlet to a tensor vari...
FAILED tests/test_step_methods.py::test_TransMatConjugateStep_subtensors - NotImplementedError: Cannot convert p_0 ~ Dirichlet to a ...
FAILED tests/test_utils.py::test_logsumexp[test_input2] - aesara.link.c.exceptions.CompileError: Compilation failed (return status=1):
FAILED tests/test_utils.py::test_tt_logdotexp - aesara.link.c.exceptions.CompileError: Compilation failed (return status=1):
FAILED tests/test_utils.py::test_multilogit_inv[test_input0-test_output0] - aesara.link.c.exceptions.CompileError: Compilation faile...
FAILED tests/test_utils.py::test_multilogit_inv[test_input1-test_output1] - aesara.link.c.exceptions.CompileError: Compilation faile...
FAILED tests/test_utils.py::test_multilogit_inv[test_input2-test_output2] - aesara.link.c.exceptions.CompileError: Compilation faile...
FAILED tests/test_utils.py::test_multilogit_inv[test_input3-test_output3] - aesara.link.c.exceptions.CompileError: Compilation faile...
============================================= 21 failed, 5 passed, 47 warnings in 39.59s ==============================================

Looks like there are a few that happened in different tests. How should we go about fixing some of these?