Remove index to access descr (#528)

* Remove the index_access_descr from Einsum object.

* Ruff corrections.

* Make the index_to_access_descriptor a cached property that will automatically build a dictionary for strings to access descriptor. Note that this may not be the exact same string as what the user passed in pt.einsum, but will be equivalent.

* Add a comment on the new cached property.

* Fixing comments made by ruff.

* Remove the cached property. Add in a warning to users requesting a subscript string.

* Remove duplicated code in generating a einsum string. The numpy code generator returned a string which contained spaces after the punctuation. It now no longer does.

* Numpy code generator seems to be dependent on the spaces after the punctuation. So, I am adding those back in.

* Deprecate the old function get_einsum_subscript_str, in favor of get_einsum_specification. Update the code to use the new function.

* Visualization needed to be updated as well.

* Update pytato/utils.py

Co-authored-by: Andreas Klöckner <inform@tiker.net>

* Update pytato/utils.py

Co-authored-by: Andreas Klöckner <inform@tiker.net>

* Updates target.

* Update the documentation to be more clear.

* Fixed a typo.

* Remove excess assert statement.

* Ensure there is a unique error message for arguments of string type since we are removing that functionality.

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Co-authored-by: Andreas Klöckner <inform@tiker.net>