Deprecate ExternalCOp

[ricardoV94]

• Inline DimShuffle C code as a plain COp
• Deprecate ExternalCOp

DimShuffle was the only one, and it paid the price. Every instantiaton READ FROM DISK! For the most common glue Op that's just outrageous.

import pytensor.tensor as pt

x = pt.tensor3("x")

def nest_expand_dims(x, n=20):
    out = x
    for _ in range(n):
        out = pt.expand_dims(out, 0)
    return out

%timeit nest_expand_dims(x)
# before (b264b264f): 8.61 ms ± 975 µs per loop
# after  (f31c4372f): 1.73 ms ± 78.6 µs per loop

It also penalized the C-backend because the generated function stayed polymorphic. Cherry picked (with some opinionated changes of mine) from #2218. Was too pissed to wait.

Deprecated the ExternalCOp. According to the bot once it's out for good roughly two-thirds of the pytensor/link/c/op.py module (~415 of 647 lines) goes away with it.

[jessegrabowski]

nice, i really like #2218, happy to see parts of it going in.