Optimize sequence_gen and uniform_sequence_gen to reduce template instantiation depth

[tenpercent]

Summary

• Use compiler builtin __make_integer_seq for sequence_gen and uniform_sequence_gen
• Reduces template instantiation depth from O(N) to O(1)

Motivation

The recursive sequence_gen_impl creates deep template chains for large sequences, increasing compile time and memory usage.

Test Plan

• Waiting for full CI

PR Stack

#	PR	Description
1	#3585	sequence_gen with __make_integer_seq
2	#3588	generate_identity_sequences helper
3	#3589	Named functors in transform_tensor_descriptor
4	#3590	container_concat optimization
5	#3596	O(1) pack expansion rewrites
6	#3600	TensorDescriptor/TensorAdaptor lambda elimination

[shumway]

Do you want to add unit tests for this, or just rely on the tests of all the code that depends on this? If it's easy to add unit tests, that's generally better, but I'm also fine with moving fast to cut down compilation times.

[shumway]

I like these specializations. It will be interesting to get a survey of the code to see how often the specializations are used and if these four smallest cases are the most impactful ones.

[tenpercent]

I'm using the build traces to drive the optimizations. Maybe removing the unused code is one other aspect which could help with parsing times

[tenpercent]

Do you want to add unit tests for this, or just rely on the tests of all the code that depends on this? If it's easy to add unit tests, that's generally better, but I'm also fine with moving fast to cut down compilation times.

let's move fast, in case something important breaks CI will catch it, tests also need maintenance and let's see how much of the metaprogramming is left after the initial sprint