users can disable the gradient computation

[yorkerlin]

No description provided.

[yorkerlin]

@karlnapf
take a look at this

[coveralls]

Changes Unknown when pulling 6e43637 on yorkerlin:inference into * on shogun-toolbox:develop*.

[karlnapf]

why is the field called "is" ?

what about "m_compute_gradients?

Minor "Whether gradients are computed" (capital W)

[karlnapf]

It is good to have this option.
BUT:

• I don't like that users have to think about this
• Gradients should be computed if model-selection is performed and otherwise not
• without the user interacting
• If users explicitly asks for gradients, should be computed lazily
• So the setter for the flag is not the most elegant way to do all this

Thoughts?

[yorkerlin]

for lazy update, it is related to this
#2838

the fact is in SGObject
The hash value is changed if parameter_hash_changed() is called even when values of variables in the object remain unchanged.

for lazy update, I need
No matter how many times parameter_hash_changed() is called, the hash value is unchanged if values of variables in the object remain unchanged.

[yorkerlin]

@karlnapf
the idea using lazy update is at #2839

I close this PR.

[karlnapf]

Ok, please do the lazy one after the bugfix. We dont want users to need to be explicit about this