Work around function std name collision in MSVC

[zcbenz]

Refs #1513.

There is a bug in MSVC that, if the std function is exposed to current scope with using namespace mlx::core, all following std::xxx invocations that try to access the namespace std will fail with very unhelpful errors:

C:\cygwin\home\zcbenz\codes\mlx\python\src\array.cpp(28,18): error C2874: using-de
claration causes a multiple declaration of 'std' [C:\cygwin\home\zcbenz\codes\mlx\
build\temp.win-amd64-cpython-311\Release\mlx.core\python\src\core.vcxproj]
      C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.42.
  34433\include\numeric(22,1):
      see declaration of 'std'
C:\Users\zcbenz\AppData\Roaming\Python\Python311\site-packages\nanobind\include\na
nobind\nb_cast.h(160,27): error C2872: 'std': ambiguous symbol [C:\cygwin\home\zcb
enz\codes\mlx\build\temp.win-amd64-cpython-311\Release\mlx.core\python\src\core.vc
xproj]
  (compiling source file '../../../../../../python/src/convert.cpp')
      C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.42.
  34433\include\numeric(22,1):
      could be 'std'
      C:\Users\zcbenz\AppData\Roaming\Python\Python311\site-packages\nanobind\incl
  ude\nanobind\nb_cast.h(160,27):
      or       'std'

(MSVC thinks std:: is ambiguous.)

There are 2 solutions, one taken by this PR is to rename the std function to std_dev and use it internally, while adding a std alias only available in public mlx.h header. It makes minimum changes to existing code with the cost of adding a new header.

The other solution would be replacing the using namespace mlx::core with namespace mx = mlx::core and use mx::xxx() instead of xxx() in the python bindings code. I prefer this one but it would result in a relatively large PR.

[awni]

I kind of prefer the second solution as well ... it feels like the right way to fix name collisions and how the C++ namespace should be used in user code. It is a big and tedious diff though 😓

[zcbenz]

Will go the 2nd solution, feels like the right way to go.