Add Einsum for CPU, Wasm and Webgl

[Hoff97]

When #223 gets merged, the tests can be added to test-suite-whitelist.jsonc
This currently has two limitations:

• For Wasm at most 2 inputs are supported. I couldnt figure out how to pass an array of tensors into wasm.
• Ellipsis (eg when the equation is "...ij -> ...i") are not supported yet