Deprecation for `_default` in FqnToConfig

[jcaip]

Previously, we allowed users to specify a _default config. With the introduction of regex and parameter matching in #3084 and #3083 respectively, we would like to deprecate this functionality in torchao in a future release.

quant_config = FqnToConfig({
    "_default": Float8DynamicActivationFloat8WeightConfig(
        granularity=PerRow(),
    ),
})

Timeline:
0.15.0: announce deprecation of _default
0.16.0: _default deprecated