[idea] fuse einsums

[Mario Geiger]

Make a transformer that detects einsums that can be fused.

Examples

These two einsum can be fuse together

a = einsum(...)
b = einsum(..., a)

but not these ones

a = einsum(...)
b = einsum(..., a)
c = einsum(..., a)

because a is used twice and we cannot ensure that repeating the operation is better.

[Alby M.]

This is now available as fuse_einsums (see tests in test_fuse.py)