[Resources] Add memory in resources

[Michaelvll]

Fixes #1673, this simulates the behavior of cpus in resources, thanks to the great effort from @WoosukKwon in #1622

Added a memory filtering field for the resources:

1. memory: 16 the memory should be 16GB
2. memory: 16+ the memory should be more than 16GB
3. memory: 4x the memory to core ratio should be larger than 4 times. (Not expose this API as it might be confusing)

Logics for choosing the CPU instance to use:

1. Use the general-purpose CPU instance by default, when the --memory is not specified

2. Use the cheapest CPU instance that satisfies the requirement, when the --memory is specified.

3. sky launch -c normal

I 03-07 21:32:21 optimizer.py:712] Considered resources (1 node):
I 03-07 21:32:21 optimizer.py:760] ------------------------------------------------------------------------------------------------
I 03-07 21:32:21 optimizer.py:760]  CLOUD    INSTANCE           vCPUs   Mem(GB)   ACCELERATORS   REGION/ZONE   COST ($)   CHOSEN   
I 03-07 21:32:21 optimizer.py:760] ------------------------------------------------------------------------------------------------
I 03-07 21:32:21 optimizer.py:760]  AWS      m6i.2xlarge        8       32        -              us-east-2     0.38          ✔     
I 03-07 21:32:21 optimizer.py:760]  Azure    Standard_D8_v5     8       32        -              eastus        0.38                
I 03-07 21:32:21 optimizer.py:760]  GCP      n2-standard-8      8       32        -              us-central1   0.39                
I 03-07 21:32:21 optimizer.py:760]  Lambda   gpu_1x_a100_sxm4   30      200       A100:1         us-east-1     1.10                
I 03-07 21:32:21 optimizer.py:760] ------------------------------------------------------------------------------------------------

2. sky launch -c cpus --cpus 32+: general purpose

I 03-07 21:33:55 optimizer.py:760] -----------------------------------------------------------------------------------------------
I 03-07 21:33:55 optimizer.py:760]  CLOUD    INSTANCE          vCPUs   Mem(GB)   ACCELERATORS   REGION/ZONE   COST ($)   CHOSEN   
I 03-07 21:33:55 optimizer.py:760] -----------------------------------------------------------------------------------------------
I 03-07 21:33:55 optimizer.py:760]  AWS      m6i.8xlarge       32      128       -              us-east-2     1.54          ✔     
I 03-07 21:33:55 optimizer.py:760]  Azure    Standard_D32_v5   32      128       -              eastus        1.54                
I 03-07 21:33:55 optimizer.py:760]  GCP      n2-standard-32    32      128       -              us-central1   1.55                
I 03-07 21:33:55 optimizer.py:760]  Lambda   gpu_4x_a6000      56      400       A6000:4        us-east-1     3.20                
I 03-07 21:33:55 optimizer.py:760] -----------------------------------------------------------------------------------------------

3. sky launch -c compute-optimized --cpus 32+ --memory 32+

I 03-07 21:43:40 optimizer.py:760] ------------------------------------------------------------------------------------------------
I 03-07 21:43:40 optimizer.py:760]  CLOUD    INSTANCE           vCPUs   Mem(GB)   ACCELERATORS   REGION/ZONE   COST ($)   CHOSEN   
I 03-07 21:43:40 optimizer.py:760] ------------------------------------------------------------------------------------------------
I 03-07 21:43:40 optimizer.py:760]  GCP      n2-highcpu-32      32      32        -              us-central1   1.15          ✔     
I 03-07 21:43:40 optimizer.py:760]  Azure    Standard_F32s_v2   32      64        -              eastus        1.35                
I 03-07 21:43:40 optimizer.py:760]  AWS      c6i.8xlarge        32      64        -              us-east-2     1.36                
I 03-07 21:43:40 optimizer.py:760]  Lambda   gpu_4x_a6000       56      400       A6000:4        us-east-1     3.20                
I 03-07 21:43:40 optimizer.py:760] ------------------------------------------------------------------------------------------------
I 03-07 21:43:40 optimizer.py:760] 

4. sky launch -c memory-optimized --memory 32+

I 03-07 21:44:13 optimizer.py:760] ------------------------------------------------------------------------------------------------
I 03-07 21:44:13 optimizer.py:760]  CLOUD    INSTANCE           vCPUs   Mem(GB)   ACCELERATORS   REGION/ZONE   COST ($)   CHOSEN   
I 03-07 21:44:13 optimizer.py:760] ------------------------------------------------------------------------------------------------
I 03-07 21:44:13 optimizer.py:760]  AWS      r6i.xlarge         4       32        -              us-east-2     0.25          ✔     
I 03-07 21:44:13 optimizer.py:760]  Azure    Standard_E4_v5     4       32        -              eastus        0.25                
I 03-07 21:44:13 optimizer.py:760]  GCP      n2-highmem-4       4       32        -              us-central1   0.26                
I 03-07 21:44:13 optimizer.py:760]  Lambda   gpu_1x_a100_sxm4   30      200       A100:1         us-east-1     1.10                
I 03-07 21:44:13 optimizer.py:760] ------------------------------------------------------------------------------------------------

Tested (run the relevant ones):

• Any manual or new tests for this PR (please specify below)
  • pytest tests/test_optimizer_dryruns.py with added memory related unit tests.
• All smoke tests: pytest tests/test_smoke.py
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: bash tests/backward_comaptibility_tests.sh

[WoosukKwon]

Great job! Thanks for the PR. Left some minor comments.