Fix PowerTransformer leaves constant feature unchanged

[jeremiedbb]

When a feature is constant, Powertransformer with method="yeo-johnson" sets an unmeaningful lambda which arbitrarily scales the feature. I think it should be left unchanged in that case, and lambda should be set to 1 because the yeo-johnson transformation with lambda=1 correponds to the identity transformation.

This also fixes a couple of failing tests in the scipy-dev job because scipy now raises an error when the optimisation of lambda failed (scipy/scipy#17704), which is the case for constant features.

(Note: this is irrelevant for method="boxcox" because it does not support constant features from the start and scipy already used to raise an informative error message)

Partial fix for #26154.

[ogrisel]

For the sake of searchability, here is the error raised on the scipy-dev error log for test_power_transformer_yeojohnson_any_input:

scipy.optimize._optimize.BracketError: The algorithm terminated without finding a valid bracket. Consider trying different initial points.

_______________ test_power_transformer_yeojohnson_any_input[X3] ________________
[gw0] linux -- Python 3.11.3 /usr/share/miniconda/envs/testvenv/bin/python

X = array([[0., 0., 0., ..., 0., 0., 0.],
            fb = fc
            fc = fw
    
        # three conditions for a valid bracket
        cond1 = (fb < fc and fb <= fa) or (fb < fa and fb <= fc)
        cond2 = (xa < xb < xc or xc < xb < xa)
        cond3 = np.isfinite(xa) and np.isfinite(xb) and np.isfinite(xc)
        msg = ("The algorithm terminated without finding a valid bracket. "
               "Consider trying different initial points.")
        if not (cond1 and cond2 and cond3):
            e = BracketError(msg)
            e.data = (xa, xb, xc, fa, fb, fc, funcalls)
>           raise e
E           scipy.optimize._optimize.BracketError: The algorithm terminated without finding a valid bracket. Consider trying different initial points.

_gold      = 1.618034
_verysmall_num = 1e-21
args       = ()
cond1      = False
cond2      = True
cond3      = True
e          = BracketError('The algorithm terminated without finding a valid bracket. Consider trying different initial points.')
fa         = inf
fb         = inf
fc         = inf
func       = <function PowerTransformer._yeo_johnson_optimize.<locals>._neg_log_likelihood at 0x7f8ec46f3560>
funcalls   = 3
grow_limit = 110.0
iter       = 0
maxiter    = 1000
msg        = 'The algorithm terminated without finding a valid bracket. Consider trying different initial points.'
xa         = -2
xb         = 2
xc         = 8.472135999999999

/usr/share/miniconda/envs/testvenv/lib/python3.11/site-packages/scipy/optimize/_optimize.py:3047: BracketError

[ogrisel]

LGTM!

[lesteve]

Merging, thanks a lot!